Keycloak.Client

HttpClient wrapper for Keycloak API

[!NOTE] Keycloak Admin REST API has an OpenAPI definition (https://www.keycloak.org/docs-api/latest/rest-api/openapi.json), but it is still not perfect (e.g. operation IDs are missing), so automatic code generation is not feasible at the moment. https://github.com/keycloak/keycloak/discussions/8898 Once this PR (https://github.com/keycloak/keycloak/pull/39011) is merged, it is worth to check code generation e.g. NSwag.