feat: use FeeRate type in TxBuilder

[reez]

Description

Use new FeeRate type in TxBuilder per #437 (comment) in #437

"For another PR, but now that we have the FeeRate type, let's use it in the TxBuilder the same way it's used in Rust bdk."

Notes to the reviewers

I think I've completed the requested changes via:

• Updated wallet.rs to use FeeRate type in TxBuilder
• Added from_sat_per_vb + from_sat_per_kwu for the ability to construct a FeeRate
• Updated tests across all 4 platforms (using the new from_sat_per_vb)

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

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

[thunderbiscuit]

ACK 8e9d2dd.