Merge bitcoindevkit/bdk#678: Implement conversion for Lightning fee rate

de358f8cdc97bf685ba46cf6751443d007b9f58e Implement conversion for Lightning fee rate (Vladimir Fomene)

Pull request description:

  This PR fixes #608.

  ### Description

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

  ### Notes to the reviewers

  Matt was concerned that we might round down value in fee calculation in such a way that a transaction may not be relayed because it is below Bitcoin Core's min relay fee (1 sat/vbyte). I don't think we need to worry about that because we [round up(ceil)](https://github.com/bitcoindevkit/bdk/blob/master/src/types.rs#L91) during fee calculation, we don't round down. I will love to hear what you think. Is there something I'm missing? @johncantrell97, I will appreciate your review on this one.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature
  * [x] I've updated `CHANGELOG.md`

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [ ] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  danielabrozzoni:
    ACK de358f8cdc97bf685ba46cf6751443d007b9f58e

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