Support unknown PSBT fields

[louisinger]

Unknown PSBT fields disappear when mergeKeyMap is executed. However, ark is using them to transport extra arguments to finalize complex inputs between users. It could also be useful to set musig2 PSBT fields before BIP373 is accepted.

From README:

We strip 'unknown' keys inside PSBT, they needed for new version/features support, however any unsupported feature/new version can significantly break assumptions about code. If you have use-case where they are needed, create a github issue.

What's required to support unknown PSBT keys ? What do you mean exactly by "break assumptions about code" ?

[paulmillr]

Fingerprinting is an action when a wallet covertly adds some garbage info to tx that allows to identify its users on blockchain.

We need to ensure user transactions aren't fingerprintable. Your pull request will allow to make them fingerprintable by default.

[louisinger]

Fingerprinting is an action when a wallet covertly adds some garbage info to tx that allows to identify its users on blockchain.

but PSBTs never hit the chain, so the garbage info will disappear when tx is extracted isn't it ?