openidc: add token_introspection_endpoint to well-known config
    
    flask-oidc expects the server metadata (found at
    .well-known/openid-configuration, and constructed by this
    method) to contain a 'token_introspection_endpoint' key in order
    to validate tokens via introspection, but we aren't setting it.
    Per @puiterwijk it's a de facto but not de jure standard, and he
    was trying to follow the spec strictly when writing it. However,
    we need this to make token auth work with waiverdb now it's
    relying on flask-oidc to validate the token via introspection,
    so let's add it. See
    https://github.com/release-engineering/waiverdb/issues/219
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>