IDTokenVerifier from Provider

[mitar]

Provider.Verifier's documentation currently states:

The returned IDTokenVerifier is tied to the Provider's context and its behavior is undefined once the Provider's context is canceled.

Looking at the code I do not see this to be true. There is no context being passed from Provider to IDTokenVerifier. There is no context even saved into Provider. The only context is inside RemoteKeySet but even that one explicitly creates a new/cloned context using context.Background().

I think this line should be removed from documentation.

[ericchiang]

I think those are a hold over from the old logic :)

Feel free to send a PR

[mitar]

Done: #336