Merge bitcoindevkit/bdk#1373: Wrap transactions as Arc<Transaction> in TxGraph

8ab58af093 feat(chain)!: wrap `TxGraph` txs with `Arc` (志宇)

Pull request description:

  ### Description

  This PR makes `TxGraph` store transactions as `Arc<Transaction>`.

  `Arc<Transaction>` can be shared between the chain-source and receiving structures. This allows the chain-source to keep the already-fetched transactions (save bandwith) and have a shared pointer to the transaction (save memory).

  Our current logic to avoid re-fetching transactions is to refer back to the receiving structures. However, this means an additional round of locking our receiving structures.

  ### Notes to the reviewers

  This will make more sense once I update the esplora/electrum chain sources to make use of both #1369 and this PR. The result would be chain sources which would only require locking the receiving structures twice (once for initiating the update, and once for applying the update).

  ### Changelog notice

  * Changed `TxGraph` to store transactions as `Arc<Transaction>`. This allows chain-sources to cheaply keep a copy of already-fetched transactions.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [ ] I've added tests for the new feature
  * [x] I've added docs for the new feature

ACKs for top commit:
  LLFourn:
    ACK 8ab58af093
  notmandatory:
    ACK 8ab58af093

Tree-SHA512: 81d7ad35fed7f253a3b902f09a41aaef2877f6201d4f6e78318741bf00e4b898a8000d878ffcbfe75701094ce3e9021bd718b190a655331a9e11f0ad66bdb02f

crates/esplora/tests/async_ext.rs

@@ -66,7 +66,7 @@ pub async fn test_update_tx_graph_without_keychain() -> anyhow::Result<()> {
     for tx in graph_update.full_txs() {
         // Retrieve the calculated fee from `TxGraph`, which will panic if we do not have the
         // floating txouts available from the transactions' previous outputs.
-        let fee = graph_update.calculate_fee(tx.tx).expect("Fee must exist");
+        let fee = graph_update.calculate_fee(&tx.tx).expect("Fee must exist");
 
         // Retrieve the fee in the transaction data from `bitcoind`.
         let tx_fee = env

crates/esplora/tests/blocking_ext.rs

@@ -80,7 +80,7 @@ pub fn test_update_tx_graph_without_keychain() -> anyhow::Result<()> {
     for tx in graph_update.full_txs() {
         // Retrieve the calculated fee from `TxGraph`, which will panic if we do not have the
         // floating txouts available from the transactions' previous outputs.
-        let fee = graph_update.calculate_fee(tx.tx).expect("Fee must exist");
+        let fee = graph_update.calculate_fee(&tx.tx).expect("Fee must exist");
 
         // Retrieve the fee in the transaction data from `bitcoind`.
         let tx_fee = env