refactor(electrum)!: put the tx cache in electrum

Previously there was a tx cache that you passed in as part of the sync
request. This seems bad and the example show'd that you should copy all
your transactions from the transaction graph into the sync request every
time you sync'd. If you forgot to do this then you would always download everything.

Instead just do a plain old simple cache inside the electrum client.
This way at least you only download transactions once. You can
pre-populate the cache with a method also and I did this in the examples.

crates/electrum/tests/test_electrum.rs

@@ -5,7 +5,7 @@ use bdk_chain::{
     spk_client::SyncRequest,
     ConfirmationTimeHeightAnchor, IndexedTxGraph, SpkTxOutIndex,
 };
-use bdk_electrum::ElectrumExt;
+use bdk_electrum::BdkElectrumClient;
 use bdk_testenv::{anyhow, bitcoincore_rpc::RpcApi, TestEnv};
 
 fn get_balance(
@@ -31,7 +31,8 @@ fn scan_detects_confirmed_tx() -> anyhow::Result<()> {
     const SEND_AMOUNT: Amount = Amount::from_sat(10_000);
 
     let env = TestEnv::new()?;
-    let client = electrum_client::Client::new(env.electrsd.electrum_url.as_str())?;
+    let electrum_client = electrum_client::Client::new(env.electrsd.electrum_url.as_str())?;
+    let client = BdkElectrumClient::new(electrum_client);
 
     // Setup addresses.
     let addr_to_mine = env
@@ -122,7 +123,8 @@ fn tx_can_become_unconfirmed_after_reorg() -> anyhow::Result<()> {
     const SEND_AMOUNT: Amount = Amount::from_sat(10_000);
 
     let env = TestEnv::new()?;
-    let client = electrum_client::Client::new(env.electrsd.electrum_url.as_str())?;
+    let electrum_client = electrum_client::Client::new(env.electrsd.electrum_url.as_str())?;
+    let client = BdkElectrumClient::new(electrum_client);
 
     // Setup addresses.
     let addr_to_mine = env