extract mcp-core (#557)

* extract mcp-core
  - This change extracts classes previously in `mcp` to `mcp-core`; a new module which does not depend on `mcp-json-jackon2`.
  - `mcp` module is now an umbrella module that brings `mcp-json-jackon2` and `mcp-core`
* add mcp-core to bom

BREAKING CHANGE: All source code in the previously `mcp` module is moved to the `mcp-core`.  the `mcp` now is just an aggregation for `mcp-core` + `mcp-json-jackson2` for backward compatibility. 
This change won't affect the end users, but will cause conflicts for the existing PRs.