feat(electrum): Add relay_fee and transaction_get_raw

[ItoroD]

Description

This concludes all APIs highlighted in #662

Notes to the reviewers

Documentation

• bdk_wallet

• bitcoin

• uniffi

• Other:

• relay_fee : https://docs.rs/electrum-client/0.24.0/electrum_client/trait.ElectrumApi.html#tymethod.relay_fee

• transaction_get_raw : https://docs.rs/electrum-client/0.24.0/electrum_client/trait.ElectrumApi.html#tymethod.transaction_get_raw

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added a changelog in the next release tracking issue (see example)
• I've linked the relevant upstream docs or specs above

New Features:

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

[thunderbiscuit]

ACK 36db4bd.