Allow subpaths in MAS endpoints

[devonh]

Fixes #19184

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[sandhose]

Good overall, but maybe we should just roll back to the old way we did that instead?

[sandhose]

I wonder if we wouldn't be better off just doing what we were doing before? e.g.

    @property
    def _metadata_url(self) -> str:
        return f"{str(self._config.endpoint).rstrip('/')}/.well-known/openid-configuration"

    @property
    def _introspection_endpoint(self) -> str:
        return f"{str(self._config.endpoint).rstrip('/')}/oauth2/introspect"

alternatively we could probably use urllib.parse.urljoin, but that might also have weird consequences depending on the trailing slash:

>>> import urllib.parse
>>> urllib.parse.urljoin("https://example.com", ".well-known/openid-configuration")
'https://example.com/.well-known/openid-configuration'
>>> urllib.parse.urljoin("https://example.com/foo", ".well-known/openid-configuration")
'https://example.com/.well-known/openid-configuration'
>>> urllib.parse.urljoin("https://example.com/foo/", ".well-known/openid-configuration")
'https://example.com/foo/.well-known/openid-configuration'

[devonh]

Hmm yeah the old way seems a lot simpler.
Not sure why it was drastically changed in the first place.

I say we go back to the old way.