Flags Feature: Enable method

Feature Flagging mobile SDK MVP spike
stacked on #2899
FFL-1117
FFL-1118

What does this PR do?

This PR implements the complete Flags feature initialization and client management APIs, bringing both Flags.enable() and FlagsClient up to the mobile API specification. The PR includes configuration management, multi-domain client support, and graceful degradation patterns.

Key Changes:

1. Flags.enable() Configuration

• Enhanced FlagsConfiguration with customExposureEndpoint
• Made configuration parameter optional with default configuration for convenience
• Updated FlagsFeature to accept and store FlagsConfiguration for future client creation
• Configuration is properly passed through to FlagsContext and network layer

2. FlagsClient Factory API

• Implemented get() method with multi-tenant support: get(name: String = "default", sdkCore: SdkCore)
  • Auto-creates default client when name == "default"
  • Returns NOPClient for custom names not found (safe fallback)
• Implemented Builder pattern for explicit client creation
  • Returns existing client with warning if duplicate requested
  • Never crashes - always returns valid FlagsClient (real or NOP)
• Renamed setContext() to setEvaluationContext() for OpenFeature alignment

3. Multi-Client Management

• Support for multiple SDK cores and multiple named clients per core
• Lifecycle management with unregister() and clear() methods

Motivation

This change fully implements the mobile API specification for Feature Flagging:

// Feature initialization
Flags.enable()                                    // default config, default core
Flags.enable(flagsConfig)                         // custom config, default core
Flags.enable(flagsConfig, core)                   // advanced, custom core

// Client access
val client = FlagsClient.get()                    // get/create default client
val customClient = FlagsClient.get("custom")      // get custom named client (or NOP)
val newClient = FlagsClient.builder("name").build()        // explicitly create named client

// Flag evaluation
client.setEvaluationContext(context)              // set targeting context
val enabled = client.resolveBooleanValue("feature-key", false)

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)