Add design proposal for K8s-aware vMCP with dynamic backend discovery

The vMCP implementation currently has duplicated logic between the operator
and vMCP server. The operator discovers backends, resolves
ExternalAuthConfigs, and embeds backend configuration into a ConfigMap.
Meanwhile, the vMCP server has its own discovery code but only loads
config statically from the ConfigMap at startup.

This duplication creates inconsistency risk, requires pod restarts for
backend changes to take effect, and has grown the operator complex with
auth resolution logic scattered across components.

This proposal flips responsibilities: the operator becomes a "dumb"
infrastructure manager handling only Deployment/Service/RBAC, while
vMCP becomes "smart" by running controller-runtime with informers to
watch MCPServer/ExternalAuthConfig/Secret resources directly.

Relates-to: #2855

Author: jhrozek