feat: adds mcp-spi

[Aliaksie]

This PR introduces the first version of the mcp-spi module to define a transport-agnostic, reactive-compatible interface for the Model Context Protocol Java SDK.

Key changes:

• New mcp-spi , transport interface + Jackson-free schema types + McpSchemaCodec abstraction
• New mcp-schema-jackson Jackson-based McpSchemaCodec implementation
• Renamed mcp to mcp-reactor;
• adapts mcp-reactor to use mcp-spi and mcp-schema-jackson (as default codec implementation)
• makes it possible to use own codec implementation for mcp-reactor
• minor changes...

Motivation and Context

How Has This Been Tested?

Breaking Changes

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

Would love a review from the team to confirm:

• Is this a good direction to start generalizing the transport layer?
• Are the abstractions in mcp-spi sensible and flexible enough?
• Any feedback on naming, structure, or patterns before we move forward?
• more questions ? ;-)

p.s. if these changes don't make sense no problem just close this pr :-)