Load feature flags from the dedicated feature flag resource endpoint

[Yuan Qu (yuanqu72)]

Summary

Loads feature flags from the new dedicated Feature Flag resource endpoint (FeatureFlagClient/FeatureFlag) in azure-appconfiguration-provider, in addition to the existing key-value based feature flags.

• When feature_flag_enabled=True, feature flags created via the feature flag resource endpoint are now loaded automatically alongside key-value based feature flags.
• Both kinds are merged into the same feature_management.feature_flags list. When a resource-based feature flag and a key-value based feature flag share the same name, the resource-based one takes precedence.
• No new load() options are required to opt in; existing feature_flag_selectors filter both kinds.
• Bumped minimum dependency on azure-appconfiguration to >=1.10.0b1 for FeatureFlagClient/FeatureFlag support.

Dependency on #47620

⚠️ This PR is stacked on top of #47620 (AppConfig-FFEndpoint branch), which introduces the FeatureFlagClient APIs in azure-appconfiguration that this change depends on. Because of that, this PR targets AppConfig-FFEndpoint instead of main — the diff here is scoped only to sdk/appconfiguration/azure-appconfiguration-provider.

This PR should be reviewed/merged after (or together with) #47620, once that lands in main.

Changes

• _azureappconfigurationprovider.py / aio/_azureappconfigurationproviderasync.py: load and merge feature flags from the feature flag resource endpoint
• _client_manager.py / aio/_async_client_manager.py: new client manager support for feature flag resource clients
• _constants.py, _request_tracing_context.py: new constants and request tracing updates for the feature flag resource path
• New samples: samples/feature_flag_resource_sample.py, samples/async_feature_flag_resource_sample.py
• New tests: tests/test_provider_feature_flag_resources.py, tests/test_async_provider_feature_flag_resources.py, tests/test_azureappconfigurationproviderbase.py, tests/test_configuration_client_manager.py
• CHANGELOG.md, README.md, samples/README.md updated

Testing

• Added unit tests and integ tests covering feature flag resource loading, merging/precedence behavior, and client manager creation (sync + async). All of them passed locally:

✅ All 360 tests passed (7m52s, 0 failures, 0 skips)

• Wrote script locally to make sure FeatureManagement Library is able to consume the json loaded from provider library for the enhanced feature flag.

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[Matthew Metcalf (mrm9084)]

/azp run

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[Matthew Metcalf (mrm9084)]

/azp run appconfiguration

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[Matthew Metcalf (mrm9084)]

/azp run python - appconfiguration

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).

[Yuan Qu (yuanqu72)]

/azp run python - appconfiguration

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).

[Yuan Qu (yuanqu72)]

Closing the pr. Moved it to the new pr: #48668