Replace bdk FeeRate with rust-bitcoin FeeRate

[notmandatory]

Describe the enhancement

We should be able to replace the bdk FeeRate struct with the FeeRate from rust-bitcoin since they serve the same purpose and it's less confusing to users to have only one version of the type. Any missing functions from the bdk::FeeRate should be upstreamed to rust-bitcoin or moved to a new bdk trait implemented on rust-bitcoin's FeeRate.

Use case

All existing uses for bdk's FeeRate.

Additional context

https://docs.rs/bitcoin/0.30.1/bitcoin/blockdata/fee_rate/struct.FeeRate.html
https://docs.rs/bdk/1.0.0-alpha.1/bdk/struct.FeeRate.html

[danielabrozzoni]

This is a duplicate of #1037, but since this is way better specified, I'm closing that one and leaving this open instead haha

[notmandatory]

We have two PRs working on resolving this issue, #1141 and #1216. I've moved them all to alpha.4 release so everyone has time to review them and we need to decide which one to close.