[rush-azure-storage-build-cache-plugin] Add custom endpoint support via storageEndpoint

[ryceg]

Summary

The Azure Blob Storage build cache plugin hardcodes the endpoint URL to https://<account>.blob.core.windows.net/, making it impossible to use with Azurite, private endpoints, or storage emulators. This PR adds an optional storageEndpoint configuration property that overrides the default, bringing the Azure plugin to parity with the S3 plugin's existing s3Endpoint support.

Details

Added a new optional storageEndpoint property that flows through all layers:

• Schema (azure-blob-storage-config.schema.json): New storageEndpoint string property with uri format
• Interface (IAzureStorageAuthenticationOptions): New optional storageEndpoint field
• Constructor (AzureStorageAuthentication): Uses storageEndpoint when provided (with trailing slash normalization), falls back to the existing default URL
• Plugin wiring (RushAzureStorageBuildCachePlugin): Passes storageEndpoint from config JSON through to the provider
• The change is fully backwards-compatible. when storageEndpoint is omitted, behavior is identical to before.

The S3 plugin already supports this pattern via s3Endpointin AmazonS3BuildCacheProvider.

How it was tested

• Added 3 unit tests covering: default endpoint when storageEndpoint is omitted, custom endpoint usage, and trailing slash normalization
• All 10 tests pass (7 existing + 3 new)
• Build completes with no warnings
• API review file updated automatically by api-extractor

Impacted documentation

• The storageEndpoint property should be documented alongside the existing azureBlobStorageConfiguration options on https://rushjs.io/pages/maintainer/build_cache/
• The JSON schema in rush-plugins/rush-azure-storage-build-cache-plugin/src/schemas/azure-blob-storage-config.schema.json was updated

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