Remove signers from bdk_wallet

[notmandatory]

Describe the enhancement

This is a tracking issue to remove signing capabilities from Wallet and provide a new way to sign and finalize PSBTs using rust-bitcoin Psbt::sign and implementing the GetKey trait for relevant types. Please comment below or mention this issue in related issues and PRs.

Use case

Safer and more flexible approaches to signing PSBTs can be done outside of the Wallet type.

Additional context

• related issues and PRs here...
• Question: Finalizing a fully signed PSBT bdk-ffi#469
• Implement GetKey for KeyMap rust-bitcoin/rust-miniscript#765
• chore: should we remove unused SignerError variants ? #93
• Create an example custom signer using hwi-async #20