Make token issuer validation optional

### Reason/Context

In some cases, Microcks' clients may receive an auth token from sources other than the Microcks' realm. Now in this case we get `Bearer error="invalid_token", error_description="An error occurred while attempting to decode the Jwt: The iss claim is not valid"`. And there is no way to fix this without total keycloak disabling. We can't just override this `spring.security.oauth2.resourceserver.jwt.issuer-uri=` in kube config, because '' is not the same as null for Spring when it creates the validator.

### Description

All we need to fix this is to make `spring.security.oauth2.resourceserver.jwt.issuer-uri` optional. There will be no breaking changes.

### Implementation ideas

Idea

Start by reviewing the implementation idea in PR #1096 and the Spring Security resource-server configuration for spring.security.oauth2.resourceserver.jwt.issuer-uri. Done means issuer validation can be omitted when the property is absent while existing behavior with a configured issuer remains unchanged.

Written by the indexing model from the issue text.