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/wallet_test_fixture.cpp

@@ -12,7 +12,7 @@ WalletTestingSetup::WalletTestingSetup(const std::string& chainName)
 {
     m_wallet.LoadWallet();
     m_chain_notifications_handler = m_node.chain->handleNotifications({ &m_wallet, [](CWallet*) {} });
-    m_wallet_client->registerRpcs();
+    m_wallet_loader->registerRpcs();
 }
 
 WalletTestingSetup::~WalletTestingSetup()