Move bdk_chain into the bdk repo 🎉

Original repository: 250b4f1dcc/bdk_chain

Co-authored-by: Steve Myers <steve@notmandatory.org>
Co-authored-by: 志宇 <hello@evanlinjin.me>
Co-authored-by: LLFourn <lloyd.fourn@gmail.com>
Co-authored-by: rajarshimaitra <rajarshi149@gmail.com>
Co-authored-by: LagginTimes <wzc110@gmail.com>
Co-authored-by: Steve Myers <steve@notmandatory.org>
Co-authored-by: Vladimir Fomene <vladimirfomene@gmail.com>

Cargo.toml

@@ -1,6 +1,7 @@
 [workspace]
 members = [
     "crates/bdk",
+    "crates/bdk_chain",
     "example-crates/esplora-wallet",
     "example-crates/electrum-wallet",
 ]