Move various SigningProviders to signingprovider.{cpp,h}

Moves all of the various SigningProviders out of sign.{cpp,h} and
keystore.{cpp,h}. As such, keystore.{cpp,h} is also removed.

Includes and the Makefile are updated to reflect this. Includes were largely
changed using:
git grep -l "keystore.h" | xargs sed -i -e 's;keystore.h;script/signingprovider.h;g'

src/wallet/wallet.cpp

@@ -13,13 +13,13 @@
 #include <interfaces/wallet.h>
 #include <key.h>
 #include <key_io.h>
-#include <keystore.h>
 #include <policy/fees.h>
 #include <policy/policy.h>
 #include <primitives/block.h>
 #include <primitives/transaction.h>
 #include <script/descriptor.h>
 #include <script/script.h>
+#include <script/signingprovider.h>
 #include <util/bip32.h>
 #include <util/error.h>
 #include <util/fees.h>