feat: Add support for SEP-2640 Skills extension

[uc4w6c]

Closes #1140

Motivation and Context

Adds support for the SEP-2640 Skills extension.

This enables Java MCP clients to discover and retrieve Agent Skills, and stateless Java MCP servers to advertise and serve them. The implementation adds the extension schema and endpoints (skills/list, skills/get, and optional resources/directory/read), client APIs, server registration APIs, manifest validation, documentation, and tests.

This aligns the Java SDK with existing implementations in:

• Python SDK: python-sdk#3485
• C# SDK: csharp-sdk#1856
• Go SDK: go-sdk#1238

How Has This Been Tested?

• Added schema serialization and deserialization tests.
• Added client tests for skill listing, skill retrieval, directory reads, and extension capability checks.
• Added stateless server tests for skill registration, endpoint handling, manifest validation, nested skills, and directory reads.
• Manually tested a local MCP server and client using the new SDK APIs.
• Ran ./mvnw clean test.

Breaking Changes

No. This change adds new APIs and protocol types without changing existing behavior.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

I prepared this implementation before realizing that the project prefers discussion with maintainers before non-trivial changes are submitted. I would appreciate feedback on the scope and API design, and I am happy to revise the implementation based on maintainer guidance.