As discussed at length in
https://github.com/modelcontextprotocol/go-sdk/issues/518, coupling our
schema fields to
jsonschema.Schema limits our user's ability to use a different schema
package, and makes it impossible for our client to talk to some
backends.
This CL relaxes the type of `*jsonschema.Schema` fields to be of type
`any`, which is a breaking change but is unlikely to affect most server
code. (It *is* a nontrivial change for clients).
Fixes https://github.com/modelcontextprotocol/go-sdk/issues/518
964574b818