Add initialize request metadata support

[smf-h]

Summary

• add optional _meta support for MCP initialize requests
• expose initialize-request metadata on async and sync client builders
• propagate metadata through lifecycle initialization while defensively copying user-provided maps

Validation

• .\mvnw.cmd -pl mcp-core "-Dtest=LifecycleInitializerTests,McpAsyncClientTest,McpSyncClientTest" test
• Result: BUILD SUCCESS; Tests run: 37, Failures: 0, Errors: 0, Skipped: 0

Fixes #940

[Kehrlann]

I'm wondering whether we should really implement #940 . If we do, we need to decide how to implement it.
Let's start the discussion there, and, if need be, we can open a PR later on.