Add RDAP search support for only contacts or only registrars

By default, RDAP entity searches return both contacts and registrars. This CL
adds a new query parameter to request only one or the other. Among other
benefits, this will allow a future CL to permit wildcard searches that return
all registrars.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181605990

java/google/registry/rdap/RdapModule.java

@@ -67,6 +67,12 @@ public final class RdapModule {
     return RequestParameters.extractOptionalParameter(req, "registrar");
   }
 
+  @Provides
+  @Parameter("subtype")
+  static Optional<String> provideSubtype(HttpServletRequest req) {
+    return RequestParameters.extractOptionalParameter(req, "subtype");
+  }
+
   @Provides
   @Parameter("includeDeleted")
   static Optional<Boolean> provideIncludeDeleted(HttpServletRequest req) {