scripted-diff: Rename interfaces::WalletClient to interfaces::WalletLoader

Name has been confusing since it was introduced, and it was pointed in
recent review club as https://bitcoincore.reviews/10102 that it was
particularly unclear how interfaces::WalletClient was different from
interfaces::Wallet.

-BEGIN VERIFY SCRIPT-
ren() { git grep -l "$1" src | xargs sed -i "s/$1/$2/g"; }
ren WalletClient WalletLoader
ren walletClient walletLoader
ren wallet_client wallet_loader
ren "wallet clients release the wallet" "wallet pointer owners release the wallet"
ren "wallet client" "wallet loader"
ren "Wallet client" "Wallet loader"
-END VERIFY SCRIPT-

src/wallet/test/init_test_fixture.cpp

@@ -11,7 +11,7 @@
 
 InitWalletDirTestingSetup::InitWalletDirTestingSetup(const std::string& chainName) : BasicTestingSetup(chainName)
 {
-    m_wallet_client = MakeWalletClient(*m_node.chain, *Assert(m_node.args));
+    m_wallet_loader = MakeWalletLoader(*m_node.chain, *Assert(m_node.args));
 
     std::string sep;
     sep += fs::path::preferred_separator;