[Platform] Apply darwin API review changes to MTRKeypair

[bzbarsky-apple]

Reproduction steps

1. Make publicKey be a readonly property, not a "method". This needs to be done a bit carefully to avoid compile issues and leaks.
2. Switch signing functions to one function that takes an enum for what sort of signature to produce (RAW or DER).
3. Add a way to ask the delegate for a bitmask indicating which signature types it supports.

Part of #22420 and looks like it's independent of the other work going on for that issue.

Platform

darwin

Platform Version(s)

No response

Type

Manually tested with SDK

(Optional) If manually tested please explain why this is only manually tested

No response

Anything else?

No response

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[woody-apple]

Closing this, given this has shipped, and more complicated to change the API surface at this point.