Support importing additional certificates into the truststore

[leochr]

Consider allowing users to import one or more certificates into the truststore only. The default location should be customizable by the user by an environment variable (i.e. TRUSTED_CERTS_DIR).

Note: At the moment, Liberty official images support importing CA cert (ca.crt) into the truststore as part of the manage TLS support, but requires tls.key and tls.crt to be specified. This option would provide flexibility to import certs into truststore only without having to provide tls.key and tls.crt.