scripted-diff: rename CBasicKeyStore to FillableSigningProvider

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

doc/developer-notes.md

@@ -375,7 +375,7 @@ reported in the debug.log file.
 
 Re-architecting the core code so there are better-defined interfaces
 between the various components is a goal, with any necessary locking
-done by the components (e.g. see the self-contained `CBasicKeyStore` class
+done by the components (e.g. see the self-contained `FillableSigningProvider` class
 and its `cs_KeyStore` lock for example).
 
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