Add support for Keycloak 8

[sberyozkin]

Describe the bug
This error is reported:

2019-11-28 13:58:50,271 WARN  [org.key.ada.KeycloakDeployment] (executor-thread-1) Failed to load URLs from http://localhost:8180/auth/realms/quarkus/.well-known/openid-configuration: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `org.keycloak.protocol.oidc.representations.OIDCConfigurationRepresentation` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)
 at [Source: (org.apache.http.conn.EofSensorInputStream); line: 1, column: 2]
	at com.fasterxml.jackson.databind.DeserializationContext.reportBadDefinition(DeserializationContext.java:1452)
	at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1028)
	at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1297)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:326)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:159)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4014)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3071)
	at org.keycloak.util.JsonSerialization.readValue(JsonSerialization.java:94)
	at org.keycloak.util.JsonSerialization.readValue(JsonSerialization.java:79)
	at org.keycloak.adapters.KeycloakDeployment.getOidcConfiguration(KeycloakDeployment.java:224)
	at org.keycloak.adapters.KeycloakDeployment.resolveUrls(KeycloakDeployment.java:178)
	at org.keycloak.adapters.KeycloakDeployment.getRealmInfoUrl(KeycloakDeployment.java:232)
	at org.keycloak.adapters.rotation.AdapterTokenVerifier.createVerifier(AdapterTokenVerifier.java:107)
	at org.keycloak.adapters.rotation.AdapterTokenVerifier.verifyToken(AdapterTokenVerifier.java:47)
	at org.keycloak.adapters.authorization.KeycloakAdapterPolicyEnforcer.requestAuthorizationToken(KeycloakAdapterPolicyEnforcer.java:172)
	at org.keycloak.adapters.authorization.KeycloakAdapterPolicyEnforcer.isAuthorized(KeycloakAdapterPolicyEnforcer.java:65)
	at org.keycloak.adapters.authorization.AbstractPolicyEnforcer.authorize(AbstractPolicyEnforcer.java:118)
	at io.quarkus.keycloak.pep.runtime.KeycloakPolicyEnforcerAuthorizer.apply(KeycloakPolicyEnforcerAuthorizer.java:43)
	at io.quarkus.keycloak.pep.runtime.KeycloakPolicyEnforcerAuthorizer.apply(KeycloakPolicyEnforcerAuthorizer.java:28)
	at io.quarkus.vertx.http.runtime.security.HttpAuthorizer$1$1.run(HttpAuthorizer.java:65)

Please follow #5822 for more information.