chore: update RMCP version to latest (#328)

This commit updates the RMCP dependency to the latest version 0.6.4.
Sadly, schemars was also updated with this, so a lot of unrelated
changes were needed to conform with the new stable schemars version.

Author: nicholascioli

.changesets/maint_nc_rmcp_update.md

@@ -0,0 +1,3 @@
+### chore: update RMCP dependency ([328](https://github.com/apollographql/apollo-mcp-server/issues/328))
+
+Update the RMCP dependency to the latest version, pulling in newer specification changes.