Separates handshake into own function

[enathang]

What (what did you do)
This pr separates the user data handshake, which previously was intertwined in the transceiver code, into its own function.
This userDataHandshake function is still applied within the transceiver function, so all this does is define an explicit wire interface between the handshake and transceiver (and splits up the logic accordingly, which is harder than one might think). This is pr 1 of a multi pr change:

pr 1: Separate handshake logic from transceiver logic

pr 2: separate Input and Output into inputs/outputs for handshake and transceiver. Have handshake and transceiver connected circuits, rather than handshake inside transceiver

pr 3: move handshake behind the elastic buffer and get rid of all handshake CDC logic (since everything will now be in tx domain)

There were also minor renamings and refactors, just because it seemed appropriate and made my life saner.

Why (context, issues, etc.)
#1199

Dear reviewer (anything you'd like the reviewer to pay close attention to?)
Because this is pr 1, there are a few things that aren't so tidy. Namely, the userDataHandshake signature is quite large (should be a data type, in next PR) and there are comments explaining why I'm doing certain things. Happy to discuss them, but the answer might be "because it makes PR 2 easier".

AI disclaimer (heads-up for more than inline autocomplete)
No

TODO

Cross-out any that do not apply

• [ ] Write (regression) test
• [ ] Update documentation, including docs/
• Link to existing issue