make id_token credential issuer optional

[stszap]

Currently users have to specify a random hs256 secret for oathkeeper even if they don't use id_token credential issuer. I think it would be more convenient to atomatically disable this issuer if there are no configurations for it.

Steps to reproduce:

$ docker run --rm -it -e DATABASE_URL=memory oryd/oathkeeper:v1.0.0-beta.9 serve api
FATA[0000] Unable to initialize the ID Token signing algorithm  error="The secret set in CREDENTIALS_ISSUER_ID_TOKEN_HS256_SECRET must be 32 characters long."

[aeneasr]

Yeah that makes sense - so should the default be to just not transform the credentials at all (noop)?

[stszap]

Yes, I think "noop" as the default option makes the most sense.

[aeneasr]

Sounds good!