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

# Jan.

- `Base64Uri` (azure `Base64Url`) not exposed
- `DateTimeRfc1123` not exposed
- Generic (`List`/`Map`) 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
Same behavior if explicitly set `new RequestOptions().setResponseBodyMode(ResponseBodyMode.DESERIALIZE)`

# Feb.

Most of above is resolved in 1.0.0-beta.5 clientcore.

We still have some (less common) serialize/de-serialize problem when done directly on proxy method.

1. Java `enum` (`java.lang.ClassCastException: class java.lang.String cannot be cast to class type.enums.fixed.DaysOfWeekEnum`)
2. Java `OffsetDateTime` https://github.com/Azure/azure-sdk-for-java/issues/43941
3. Java `Duration` (`Body provided doesn't match expected body,"expected":["P123DT22H14M12.011S"],"actual":["PT2974H14M12.011S"]`)
4. Java `Map` (`List` is resolved)