[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_electrum/src/main.rs

@@ -48,7 +48,7 @@ pub struct ScanOptions {
 }
 
 fn main() -> anyhow::Result<()> {
-    let (args, keymap, tracker, db) = cli::init::<ElectrumCommands, _>()?;
+    let (args, keymap, tracker, db) = cli::init::<ElectrumCommands, _, _>()?;
 
     let electrum_url = match args.network {
         Network::Bitcoin => "ssl://electrum.blockstream.info:50002",