Support credential instance reuse in WithAzureCredential DI path

[m-nash]

Fixes #56231

What

When multiple Azure clients are registered via DI with identical credential configuration, they now share the same \ConfigurableCredential\ instance. This enables MSAL token cache reuse across clients, improving resilience per Azure Identity best practices.

How

• Added \ConfigurableCredentialCache\ (internal) that caches \ConfigurableCredential\ instances keyed by a SHA256 hash of the credential config section's flattened key-value pairs
• Updated \WithAzureCredential(IClientBuilder)\ to use the cache via a \ConditionalWeakTable<IServiceCollection, ConfigurableCredentialCache>\ scoped per host builder
• Cache keys are content-based: two different config section paths with identical values produce the same key and share the credential
• Separate host builders get separate caches (no cross-host leakage)
• SHA256 hashing avoids leaking secrets in cache keys
• #if NETSTANDARD2_0\ conditional for optimal hashing APIs on modern TFs

Scope

Only the \WithAzureCredential(IClientBuilder)\ overload (DI path) is changed. The direct \ClientSettings\ overload is unchanged.

Tests

• 10 unit tests for cache key generation (arrays, nested values, ordering, empty sections)
• 16 integration tests exercising the full DI pipeline (same/different config, keyed/non-keyed clients, mixed registration, separate hosts, array configs)

[Copilot]

Pull request overview

This PR implements credential instance reuse for Azure SDK clients registered via dependency injection with identical credential configurations. By enabling MSAL token cache sharing across clients, it improves application resilience and follows Azure Identity best practices.

Changes:

• Introduced ConfigurableCredentialCache to cache ConfigurableCredential instances using SHA256-hashed, content-based keys
• Modified WithAzureCredential(IClientBuilder) to leverage per-host caching via ConditionalWeakTable
• Added comprehensive test coverage (26 tests) validating cache behavior across various scenarios

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sdk/identity/Azure.Identity/src/ConfigurableCredentialCache.cs	New internal cache class with SHA256-based key generation from configuration sections
sdk/identity/Azure.Identity/src/ConfigurationExtensions.cs	Added ConditionalWeakTable-based cache lookup in WithAzureCredential DI overload
sdk/identity/Azure.Identity/tests/ConfigurableCredentials/ConfigurableCredentialCacheTests.cs	Unit tests for cache key generation logic (10 tests)
sdk/identity/Azure.Identity/tests/ConfigurableCredentials/WithAzureCredentialTests.cs	Integration tests for full DI pipeline credential reuse (16 tests)
sdk/identity/Azure.Identity/tests/Azure.Identity.Tests.csproj	Added Microsoft.Extensions.Hosting package reference for tests