Pin BDK to fix breakage

BDK 0.28.1 patch release upgraded the `esplora-client` dependency to
0.5, which is API incompatible with the 0.4 release we use in
`lightning-transaction-sync`. In order to fix our builds, we pin BDK to
the older version until we had a chance to upgrade
`lightning-transaction-sync`.

Cargo.toml

@@ -56,7 +56,8 @@ lightning-transaction-sync = { version = "0.0.115", features = ["esplora-async-h
 #lightning-rapid-gossip-sync = { path = "../rust-lightning/lightning-rapid-gossip-sync" }
 #lightning-transaction-sync = { path = "../rust-lightning/lightning-transaction-sync", features = ["esplora-async"] }
 
-bdk = { version = "0.28.0", default-features = false, features = ["std", "async-interface", "use-esplora-async", "sqlite-bundled", "keys-bip39"]}
+# TODO: unpin BDK once we upgraded the esplora client to 0.5 everywhere.
+bdk = { version = "=0.28.0", default-features = false, features = ["std", "async-interface", "use-esplora-async", "sqlite-bundled", "keys-bip39"]}
 reqwest = { version = "0.11", default-features = false, features = ["json", "rustls-tls"] }
 rusqlite = { version = "0.28.0", features = ["bundled"] }
 bitcoin = "0.29.2"