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

• Question: Finalizing a fully signed PSBT bdk-ffi#469
• feat: implement GetKey for KeyMap and DescriptorSecretKey rust-bitcoin/rust-miniscript#851
• feat(bdk_wallet): add helper method to get the KeyMap for Psbt.sign #331
• Create an example custom signer using hwi-async #20
• additional information for sign, finalize_psbt when returning false #73
• chore: deprecate all signing related APIs
• chore: deprecate all policy related APIs (which relies in signers module)

[ValuedMammal]

Concept ACK. The signer module contains outdated code that is mostly supplanted by recent developments in rust-bitcoin, plus removing a bunch of traits should cut down on code bloat.