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.

mcp-bom/pom.xml

@@ -27,6 +27,13 @@
     <dependencyManagement>
         <dependencies>
             <!-- Core MCP -->
+            <dependency>
+                <groupId>io.modelcontextprotocol.sdk</groupId>
+                <artifactId>mcp-core</artifactId>
+                <version>${project.version}</version>
+            </dependency>
+
+            <!-- MCP -->
             <dependency>
                 <groupId>io.modelcontextprotocol.sdk</groupId>
                 <artifactId>mcp</artifactId>