feat: multi-provider support via SDK MultiProvider

[aubes]

Added

• Multi-provider support through the SDK MultiProvider. The new providers configuration key declares several providers (map of provider name => service ID, evaluated in declaration order, mutually exclusive with provider), and the strategy key selects the evaluation strategy: first_match (default), first_successful, or comparison with a required fallback provider name. Configuration is validated at compile time (exclusivity, fallback consistency, case-insensitive provider name uniqueness).
• The profiler panel and toolbar now display the evaluation strategy and the sub-providers in evaluation order (with a fallback badge for the comparison strategy) when multi-provider is configured.

Changed

• open-feature/sdk requirement raised from ^2.0 to ^2.2 (the bundle relies on the MultiProvider classes introduced in SDK 2.2.0).
• The provider configuration node now defaults to null instead of InMemoryProvider. When neither provider nor providers is set, the bundle still falls back to the built-in InMemoryProvider: behavior is unchanged, only the output of config:dump-reference open_feature differs.