feat(spi): adds implementation-neutral SPI module

[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.

Refactor mcp to optionally log via a pluggable logger interface (McpTransportLogger interface in mcp-spi instead of hard SLF4J dependency).
Key changes:

• New mcp-spi (transport interfaces, Jackson-free schema types, McpSchemaCodec abstraction, McpClient/McpServer abstraction, McpLogger pluggable logger interface)
• New mcp-schema-jackson Jackson-based schema + McpSchemaCodec implementation
• New mcp-logging-slf4j slf4j-based implementation of McpLogger
• Renamed mcp to mcp-reactor
• adapts mcp-reactor to use mcp-spi and mcp-schema-jackson (as default codec implementation) and mcp-logging-slf4j (as default McpLogger implementation)
• makes it possible to use own codec implementation and logger for mcp-reactor
• minor changes...

Motivation and Context

The motivation behind this change is to make the SDK modular, implementation-agnostic, and easier to integrate with diverse runtime environments.

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

Possible Future Work:

• Introduce TCK (Technology Compatibility Kit) for validating McpClientTransport and McpMessageCodec implementations (mcp-spi-tck).
• Add support for other runtime environments, e.g.(Vert.x-based transport(mcp-vertx), Java 11+ Flow.Publisher HTTP client(mcp-flow)
• Provide alternative serialization support (e.g., mcp-schema-gson, mcp-schema-cbor, mcp-schema-protobuf).
• Allow ServiceLoader-based discove.

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

[Aliaksie]

Hey @tzolov, hope you're doing well! Just wanted to check in — have you had a chance to review this PR? I’d appreciate any feedback when you get a moment. Looking forward to your thoughts!