load feature flags from new endpoint

[linglingye001]

Summary
Adds support for loading feature flags from new endpoint (via FeatureFlagClient.listFeatureFlags), in addition to the classic key-value path. When feature flags are enabled, the provider now loads flags from both sources and merges them, with flags from the new endpoint taking precedence over classic ones of the same name. Reference Azure/AppConfiguration-DotnetProvider#738

Key changes

• Added appConfigClient.ts: an IAppConfigurationClient interface plus AppConfigClient implementation that wraps both AppConfigurationClient (classic KV) and FeatureFlagClient (new FF endpoint). Each method applies request tracing before delegating to the underlying SDK client.

• Added featureFlagConverter.ts with convertToMicrosoftSchema(), which maps the SDK's typed FeatureFlag (camelCase) into the Microsoft Feature Management schema consumed by feature_management.feature_flags.

• Load and merge on init and on refresh: #loadClassicFeatureFlags() + #loadFeatureFlags() → #setFeatureFlags() dedups by name (new endpoint supersedes classic).

• Change detection covers both paths