[bdk_chain_redesign] Introduce BlockAnchor trait

* Introduce `GraphedTx` struct to access transaction data of graphed
  transactions.
* Ability to insert/access anchors and "seen at" values for graphed
  transactions.
* `Additions` now records changes to anchors and last_seen_at.

example-crates/keychain_tracker_esplora/src/main.rs

@@ -49,7 +49,7 @@ pub struct ScanOptions {
 }
 
 fn main() -> anyhow::Result<()> {
-    let (args, keymap, keychain_tracker, db) = cli::init::<EsploraCommands, _>()?;
+    let (args, keymap, keychain_tracker, db) = cli::init::<EsploraCommands, _, _>()?;
     let esplora_url = match args.network {
         Network::Bitcoin => "https://mempool.space/api",
         Network::Testnet => "https://mempool.space/testnet/api",