Support for path convertor {value:path} in FastMCP

[dsp-ant]

Currently we do not allow for matching paths in resource templates, meaning that

@app.resource("test://{foo}")

will not match test://foo/bar. This is intended. However at times people want to explicitly match paths. FastAPI offers in this case a path convertor, such that

@app.resource("test://{foo:path}")

would match test://foo/bar.

We should consider adding a convertor approach or something similar to resource templates.