refactor(split): allow central to define connection parameters

[xudongzheng]

Fixes #1614

[petejohanson]

Code seems reasonable. Has this tested well without any issues, in particular if you fail to flash both sides, and the peripheral still requests different params?

[xudongzheng]

In the event that the central is updated to include these options (and kept on the defaults) but peripheral continues running the old firmware, nothing really changes. It's the same as before except central is now getting the initial connection parameters from ZMK_SPLIT_BLE_PREF_* instead of having it hardcoded in central.c.

If the parameters are changed from the defaults on central but peripheral continues running the old firmware: central will initiate a connection with the new custom parameters but peripheral will request the old defaults. In theory central has the ability to reject that request but that seems like unnecessary code that will eventually serve no purpose.

[petejohanson]

This will need rebasing now that #836 has been merged. There's also some CI failures that hopefully will be addressed when you do so. Thanks again.

[petejohanson]

Thanks!