[backport#16227 4/8] scripted-diff: rename CBasicKeyStore to FillableSigningProvider

Summary:
-BEGIN VERIFY SCRIPT-
git grep -l "CBasicKeyStore" | xargs sed -i -e 's/CBasicKeyStore/FillableSigningProvider/g'
-END VERIFY SCRIPT-

https://github.com/bitcoin/bitcoin/pull/16227/commits/d9becff4e13da8e182631baa79b9794c03d44434

Depends on D6555

Partial backport of Core PR16227

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6556