Implement FillPSBT in DescriptorScriptPubKeyMan

Summary:
FillPSBT will add our own scripts to the PSBT if those inputs are ours.
If an input also lists pubkeys that we happen to know the private keys
for, we will sign those inputs too.

Backport of Core PR16528 [30/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/72a9540df96ffdb94f039b9c14eaacdc7d961196

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8451