create custom claim in Keycloak - data to the given claim shall be taken from the other service

[maciejgz]

Add custom claim to Keycloak where data to this claim will be get from the external API and will be retrieved optionally in the Keycloak.
Services shall use this claim to create custom authorities in Spring Security.

[maciejgz]

Possible solution to review: https://medium.com/@pavithbuddhima/how-to-add-custom-claims-to-jwt-tokens-from-an-external-source-in-keycloak-52bd1ff596d3