Add get_fee_estimates to EsploraClient

[reez]

Description

Add get_fee_estimates on EsploraClient

Notes to the reviewers

Similar method was added for Electrum in #641 and I believe this is the closest comp for Esplora so I wanted to add it.

Can run test locally similar to what thunder laid out here by import org.bitcoindevkit.EsploraClient then adding something like:

val feeEstimates = client.getFeeEstimates()
println("Fee estimates: $feeEstimates")

Changelog notice

Added:
  - EsploraClient::get_fee_estimates [#648]

[#648]: https://github.com/bitcoindevkit/bdk-ffi/pull/641

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 da9027e.

Nice use of the record type in UDL!