Back-End: Option to disable trust manager when invoking KC endpoints

[EricWittmann]

When the user unlinks a linked account, we make an http request to Keycloak to delete the identity provider link. If the URL to Keycloak is a https and the cert is e.g. self-signed, this will fail. Need to provide an option to disable the trust manager for this connection.

[EricWittmann]

Additionally, when we try to acquire an access token for a linked account (stored in KC), this error is hit. Basically any time the Apicurio back-end acts as an HTTP Client to Keycloak, we need to be able to (optionally) disable the trust manager.