feat(orchestrator): build extension profiles from configuration

## Summary

### Why?

Which implementation of each extension a queue resolved to was hardcoded: the build runner was the fake one for every queue regardless of what was available, and the change provider was a global all-or-nothing environment gate. A deployment could not run one queue against a real provider next to one running entirely on fakes, which is what a stack serving both a test environment and a live repository needs.

### What?

`PROFILES_CONFIG_PATH` names a YAML file selecting the change provider, build runner, and conflict analyzer per queue. Each extension is independently optional, so a queue that differs only in its analyzer says only that. `kind` is an open string rather than a closed schema, so supporting a new provider is a new value and an implementation behind it, not a change to the file's shape.

The file holds no secret: each integration names the environment variable carrying its credential.

With no config file the built-in example topology applies, reproducing the previous behavior exactly — including the per-queue analyzers the E2E suite depends on, and a routing change provider that still falls back to the fake when no token is set. That is what keeps the existing suite meaningful as a regression gate.

Extensions are reused across queues configured alike. This is load-bearing for the build runner: the build and buildsignal controllers look it up separately and the fake holds a build's outcome in memory, so two instances would lose the result between triggering a build and polling it.

## Test Plan

✅ `bazel test //service/submitqueue/orchestrator/server:go_default_test` — pins the built-in topology against what the E2E suite expects, covers per-extension inheritance, provider defaults, every validation rejection, that queues configured alike share one build runner, that each queue's analyzer behaves as configured, and that a missing token fails at startup rather than mid-merge.

# Conflicts:
#	service/submitqueue/orchestrator/server/main.go

# Conflicts:
#	service/submitqueue/orchestrator/server/main.go
#	service/submitqueue/orchestrator/server/profiles.go

Author: behinddwalls

service/submitqueue/orchestrator/server/BUILD.bazel

@@ -1,4 +1,4 @@
-load("@rules_go//go:def.bzl", "go_binary", "go_cross_binary", "go_library")
+load("@rules_go//go:def.bzl", "go_binary", "go_cross_binary", "go_library", "go_test")
 
 exports_files(
     ["docker-compose.yml"],
@@ -8,6 +8,7 @@ exports_files(
 go_library(
     name = "orchestrator_lib",
     srcs = [
+        "config.go",
         "main.go",
         "profiles.go",
     ],
@@ -17,6 +18,8 @@ go_library(
         "//api/submitqueue/orchestrator/protopb:go_default_library",
         "//platform/errs/generic:go_default_library",
         "//platform/errs/mysql:go_default_library",
+        "//platform/extension/buildrunner/buildkite:go_default_library",
+        "//platform/extension/buildrunner/githubactions:go_default_library",
         "//platform/extension/consumergate:go_default_library",
         "//platform/extension/consumergate/file:go_default_library",
         "//platform/extension/consumergate/noop:go_default_library",
@@ -28,7 +31,9 @@ go_library(
         "//submitqueue/core/changeset:go_default_library",
         "//submitqueue/entity:go_default_library",
         "//submitqueue/extension/buildrunner:go_default_library",
+        "//submitqueue/extension/buildrunner/buildkite:go_default_library",
         "//submitqueue/extension/buildrunner/fake:go_default_library",
+        "//submitqueue/extension/buildrunner/githubactions:go_default_library",
         "//submitqueue/extension/changeprovider:go_default_library",
         "//submitqueue/extension/changeprovider/fake:go_default_library",
         "//submitqueue/extension/changeprovider/github:go_default_library",
@@ -53,6 +58,7 @@ go_library(
         "//submitqueue/orchestrator:go_default_library",
         "@com_github_go_sql_driver_mysql//:go_default_library",
         "@com_github_uber_go_tally//:go_default_library",
+        "@in_gopkg_yaml_v3//:go_default_library",
         "@org_golang_google_grpc//:go_default_library",
         "@org_golang_google_grpc//reflection:go_default_library",
         "@org_golang_x_oauth2//:go_default_library",
@@ -87,3 +93,17 @@ filegroup(
     ],
     visibility = ["//test:__subpackages__"],
 )
+
+go_test(
+    name = "go_default_test",
+    srcs = ["config_test.go"],
+    embed = [":orchestrator_lib"],  # keep
+    deps = [
+        "//submitqueue/entity:go_default_library",
+        "//submitqueue/extension/conflict:go_default_library",
+        "@com_github_stretchr_testify//assert:go_default_library",
+        "@com_github_stretchr_testify//require:go_default_library",
+        "@com_github_uber_go_tally//:go_default_library",
+        "@org_uber_go_zap//zaptest:go_default_library",
+    ],
+)