autodecode base64 encoded uuid userid

[butonic]

Keycloak base64 encodes the AD binary objectGUID. We can automatically detect that. No need for a new env var.

[micbar]

@rhafer @butonic

I added a test for that.

Not a beauty, but the mock User Backend now validates the given claim value during "GetUserByClaim"

[rhafer]

@butonic @micbar Is there any documentation for this "feature" in Keycloak e.g. under which circumstances a value is considered "binary"?

[butonic]

superseded by #1901