Implement conversion for Lightning fee rate

Lightning denotes transaction fee rate
sats / 1000 weight units and sats / 1000 vbytes.
Here we add support for creating BDK fee rate from
lightning fee rate. We also move all FeeRate test to
types.rs and rename as_sat_vb to as_sat_per_vb.

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+- Add capacity to create FeeRate from sats/kvbytes and sats/kwu.
+- Rename `as_sat_vb` to `as_sat_per_vb`. Move all `FeeRate` test to `types.rs`.
 
 ## [v0.21.0] - [v0.20.0]
 
@@ -19,6 +21,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - New `RpcBlockchain` implementation with various fixes.
 - Return balance in separate categories, namely `confirmed`, `trusted_pending`, `untrusted_pending` & `immature`.
 
+
 ## [v0.20.0] - [v0.19.0]
 
 - New MSRV set to `1.56.1`