Download from alternate URL with optional http authentication

[guidograzioli]

Variable	Description	Required
keycloak_quarkus_alternate_download_url	Alternate location with optional authentication for downloading RHBK	no
keycloak_quarkus_download_user	Optional username for http authentication	no*
keycloak_quarkus_download_pass	Optional password for http authentication	no*
keycloak_quarkus_download_validate_certs	Whether to validate certs for URL keycloak_quarkus_alternate_download_url (if omitted, do validate)	no

* username/password authentication credentials must be both declared or both undefined

Fix #219