feat: add APIM costing & showback sample (#138)

[ncheruvu-MSFT]

Summary

Adds a new costing & showback sample that demonstrates how to track and allocate API costs using Azure API Management with Azure Monitor, Application Insights, Log Analytics, and Cost Management.

Changes

• New sample: samples/costing/ with full notebook, Bicep templates, workbook, and clean-up
• Cost Management export via managed identity (no storage key access needed)
• Azure Monitor Workbook with cost allocation, usage analytics, and response code analysis
• Screenshots for cost reports and dashboard views
• README with configuration options, prerequisites, and screenshot gallery
• Shared updates: Minor updates to azure_resources.py and template notebook

How to Test

1. Deploy any infrastructure (e.g. simple-apim)
2. Open samples/costing/create.ipynb and Run All
3. Verify workbook deploys and displays cost data

[github-actions]

🐍 Python 3.14 Results

Metric	Status	Value
Pylint Score	✅	10.00/10
Unit Tests	✅	success
Code Coverage	📊	100%

Full Workflow Logs

[github-actions]

Python 3.14 Detailed Test Results

1 458 tests  ±0   1 458 ✅ ±0   11s ⏱️ -4s
    1 suites ±0       0 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit b4df83d. ± Comparison against base commit bb5f8b4.

[github-actions]

🐍 Python 3.12 Results

Metric	Status	Value
Pylint Score	✅	10.00/10
Unit Tests	✅	success
Code Coverage	📊	100%

Full Workflow Logs

[github-actions]

🐍 Python 3.13 Results

Metric	Status	Value
Pylint Score	✅	10.00/10
Unit Tests	✅	success
Code Coverage	📊	100%

Full Workflow Logs

[github-actions]

Python 3.12 Detailed Test Results

1 458 tests  ±0   1 458 ✅ ±0   15s ⏱️ ±0s
    1 suites ±0       0 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit b4df83d. ± Comparison against base commit bb5f8b4.

[github-actions]

Python 3.13 Detailed Test Results

1 458 tests  ±0   1 458 ✅ ±0   14s ⏱️ -1s
    1 suites ±0       0 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit b4df83d. ± Comparison against base commit bb5f8b4.