Add support for ext-data-control

[YaLTeR]

Since the protocol is more or less exactly the same as wlr-data-control, I added some wrappers and macros to re-use the same existing code for both protocols.

Some things I didn't do because it seemed like too much effort:

1. Would be nice to add new traits over the types in data_control instead of macros to reduce the curse and to avoid duplicate codegen.
2. I didn't add copy and paste tests over ext-data-control; don't think it's worth the effort since it would be using the same server-side impl code anyway.

The only (if I didn't miss something) API breaking change is that I got rid of name in the MissingProtocol error and instead spelled it out in the message. This is not strictly required, but might as well do it, I'll bump minor for this anyway.

@bugaevc wanna take a look? I tested this on niri v25.02, verifying via WAYLAND_DEBUG that it used ext-data-control.

The diff is best viewed ignoring whitespace.