feat: Add RequestIdGenerator for custom request ID generation

[rhtnr]

Summary

Fixes #703

Adds support for configuring custom request ID generators, enabling compatibility with MCP servers that require specific ID formats (e.g., numeric-only IDs like the Snowflake MCP server).

Changes

• Added RequestIdGenerator functional interface in io.modelcontextprotocol.spec with:
  • generate() method for creating unique request IDs
  • ofDefault() factory method for UUID-prefixed incrementing IDs (existing behavior)
  • ofIncremental() factory method for simple numeric IDs
• Updated McpClientSession to accept a custom RequestIdGenerator
• Added requestIdGenerator() builder methods to both McpClient.SyncSpec and McpClient.AsyncSpec
• Added 3 comprehensive tests for custom ID generators

Usage Example

// Using the built-in numeric ID generator
McpClient.sync(transport)
    .requestIdGenerator(RequestIdGenerator.ofIncremental())
    .build();

// Using a custom generator
AtomicLong counter = new AtomicLong(0);
McpClient.async(transport)
    .requestIdGenerator(() -> String.valueOf(counter.incrementAndGet()))
    .build();

Backward Compatibility

This change is fully backward compatible:

• Existing code continues to work without changes
• Default behavior remains the same (UUID-prefixed incrementing IDs)
• New requestIdGenerator() method is optional

Test plan

• All 3 new tests pass for custom ID generators
• All 89 existing schema and session tests pass
• Project compiles successfully

[He-Pin]

This is actually a nice addition, thanks.