Replace GetSigningProvider with GetSolvingProvider

Summary:
Not all ScriptPubKeyMans will be able to provide private keys,
but pubkeys and scripts should be. So only provide public-only
SigningProviders, i.e. ones that can help with Solving.

Backport of Core PR18115 part [8/9] : https://github.com/bitcoin/bitcoin/pull/18115/commits/dc174881ad8498a6905ba282a48077bc5c8037a7

Depends on D8100 and D8102

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8114