[FEATURE REQ] clientcore, missing feature in http-specs

[weidongxu-microsoft]

• Base64Uri (azure Base64Url) not exposed
• DateTimeRfc1123 not exposed
• Generic (List<Model>/Map<String, Model>) and ExpandableEnum as method return is not correctly de-serialized
  • Generic https://github.com/microsoft/typespec/blob/main/packages/http-client-java/generator/http-client-generator-clientcore-test/src/test/java/type/array/ModelValueClientTest.java#L17
  • ExpandableEnum https://github.com/microsoft/typespec/blob/main/packages/http-client-java/generator/http-client-generator-clientcore-test/src/test/java/type/enums/extensible/ExtensibleClientTest.java#L15