FEAT: Allow multivalue for Config attibutes

[ChrisHubinger]

#548

Implemented & documented the same multivalue behavior for keycloak_custom_user_federation resource as it's done in keycloak_user.

[ChrisHubinger]

Unfortunately, I could not get the local test setup running with the custom user storage SPI.

The docker-compose mapps the built SPI to the /opt/jboss/keycloak/standalone/deployments/ folder, although it is already using the quarkus version of keycloak.

Just setting the correct path did not work as it throws the following build error during keycloak start up:

terraform-provider-keycloak-keycloak-1  | Updating the configuration and installing your custom providers, if any. Please wait.
terraform-provider-keycloak-keycloak-1  | The DelayedHandler was closed before any children handlers were configured. Messages will be written to stderr.
terraform-provider-keycloak-keycloak-1  | 2022-11-04 11:39:06,327 DEBUG [org.jboss.logging] (main) Logging Provider: org.jboss.logging.JBossLogManagerProvider

[..]
terraform-provider-keycloak-keycloak-1  | 2022-11-04 11:39:07,362 DEBUG [io.quarkus.deployment.QuarkusAugmentor] (main) Beginning Quarkus augmentation
terraform-provider-keycloak-keycloak-1  |
terraform-provider-keycloak-keycloak-1  | ERROR: Failed to run 'build' command.
terraform-provider-keycloak-keycloak-1  | ERROR: org.eclipse.microprofile.config.spi.ConfigSource: org.jboss.resteasy.microprofile.config.ServletConfigSource not a subtype
terraform-provider-keycloak-keycloak-1  | For more details run the same command passing the '--verbose' option. Also you can use '--help' to see the details about the usage of the particular command.
terraform-provider-keycloak-keycloak-1 exited with code 1

I'd be happy to wirte tests if I find a way to run them.

[mrparkers]

Unfortunately, I could not get the local test setup running with the custom user storage SPI.

No problem, these tests don't run in CI anyways.

[mrparkers]

Thanks for the PR!