[Bug]: http-client-java, unbranded, response as wired type Base64, Base64Uri

### Describe the bug

azure supports this in RestProxy as

```
@ReturnValueWireType(Base64Url.class)
Mono>> getBase64Url(@HostParam("$host") String host, @HeaderParam("Accept") String accept,
Context context);
```
`@ReturnValueWireType` would handle the de-serialization.

---

In clientcore, `Base64Uri` would work after we add `returnValueWireType = Base64Uri.class`.

However, `byte[]` with Base64 encode is not handled.

### Reproduction

http-specs test

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/Microsoft/typespec/discussions).
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.

Start with the http-specs test and compare its behavior with the RestProxy example using @ReturnValueWireType. Read the clientcore Base64Uri handling, then trace how a byte[] response with Base64 encoding is deserialized. Done means the reproduction test passes for both Base64Uri and Base64 responses.

Written by the indexing model from the issue text.