Support token exchange in gatekeeper

[abstractj]

What:

h3. What

Add support for token exchange to keycloak-gatekeeper (described [here|https://www.keycloak.org/docs/latest/securing_apps/index.html#_token-exchange]).

h3. Why

Currently gatekeeper rejects requests if the provided token does not contain the right audience. Since keycloak supports token exchange for changing/downgrading the current bearer token gatekeeper could try to change the token to the right audience. In this way gatekeeper could transparently support the described use cases of token exchange (for example internal token to internal token) for the requester.
If required the feature could be activated by configuration.

Reference:

• KEYCLOAK-12171 - Support token exchange in gatekeeper

[a1tan]

Is there an improvement on this feature?

[abstractj]

@a1tan please check #683