[WIP] Streamable HTTP

[ZachGerman]

Adding Streamable HTTP support

Motivation and Context

Wanting to get Java into parity with TS and Python. Plan to keep working on this until completion.
Please reach out to me via Github or the community Discord for collaborative purposes!

How Has This Been Tested?

Added some tests for server transport. Testing client against mcp-everything server.

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 - New tests pass. 15 existing abstract tests are broken for client (to be addressed)
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Next, I'm focusing on initialization with proper backward compatibility detection and support.
Then, various spec Transport and Session class changes.
Then, origin header verification.
Lastly, more new tests and test changes to accommodate new transport.