The OpenID Connect specification defines a number of ways for a
client (RP) to authenticate itself to the OP when accessing the
Token Endpoint. We currently only support `client_secret_basic`.

This change introduces support for 2 additional authentication
methods, namely `client_secret_post` (where the client credentials
are passed in the body of the POST request to the OP) and
`client_secret_jwt` where the client constructs a JWT and signs
it using the the client secret as a key.

Support for the above, and especially `client_secret_jwt` in our
integration tests meant that the OP we use ( Connect2id server )
should be able to validate the JWT that we send it from the RP.
Since we run the OP in docker and it listens on an ephemeral port
we would have no way of knowing the port so that we can configure
the ES running via the testcluster to know the "correct" Token
Endpoint, and even if we did, this would not be the Token Endpoint
URL that the OP would think it listens on. To alleviate this, we
run an ES single node cluster in docker, alongside the OP so that
we can configured it with the correct hostname and port within
the docker network.

…ent-auth-types

…ent-auth-types

…ent-auth-types

From
breskeby@de079d6