[Server][Conformance] Handler for Resource Subscribe/Unsubscribe missing

[chr-hertel]

Currently, the support for resource subscribe/unsubscribe is still missing, see #9.
Therefore, scenario resources-subscribe and resources-unsubscribe fail.

Reproduce:

make conformance-server

npx @modelcontextprotocol/conformance server --url http://localhost:8000 --scenario resources-subscribe
npx @modelcontextprotocol/conformance server --url http://localhost:8000 --scenario resources-unsubscribe