Support JWE identity tokens

[kaol]

The identity provider I'm using (Telia) returns JWE decrypted responses. openid-connect doesn't know how to proceed when that happens. This will require support from jose library before openid-connect can really do anything about them.

I have a pull request to jose that'll add (at least preliminary) support for this use (frasertweedale/hs-jose#117).

I think it'd be premature to make pull requests to openid-connect that are dependent on other projects' updates, but the corresponding changes to it are available at https://github.com/KSF-Media/openid-connect/tree/jwe-decrypt-identity-token

[pjones]

@kaol Once you get your changed merged into jose I'll be happy to accept a pull request.