make PropKey methods public

[zoedberg]

This PR makes PropKey::mpc_entropy() public, needed to avoid manually recreating the proprietary key using constants before converting it to a rust-bitcoin proprietary key.

[codecov[bot]]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (28d3228) 0.0% compared to head (48df55f) 0.0%.

Files	Patch %	Lines
psbt/src/csval/mpc.rs	0.0%	5 Missing :warning:

Additional details and impacted files

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[zoedberg]

Ok, I will update the PR then

[zoedberg]

Done