Remove signers from `Wallet`

[notmandatory]

Describe the enhancement

This is a tracking issue to remove signing capabilities and requirements to handle private key material from Wallet. The new recommended flow will leverage rust-bitcoin Psbt::sign. Please comment below or mention this issue in related issues and PRs.

Proposed timeline

When	What
Available now since 3.1.0	Allow users to bring in their private keys at the last minute while still using our APIs (Wallet::sign_with_signers).
Available now (but)	Make sure users can sign using Psbt::sign.
Coming up (bookofbdk)	Write documentation, recommendations, release notes, etc.
?	Mark signing-related APIs as deprecated.
4.0 (Option 1)	Remove signers entirely from the Wallet type and crate.
4.0 (Option 2)	Remove signers entirely from the Wallet, but keep them as separate helper module if required (after deep dive into SignOptions, see todos below).

Todos

• Add an API to keep using the wallet Signers but bring in your private keys just at the moment of signing (not in the constuction of the Wallet)
• Produce extensive documentation and write-ups on the migration, timeline, and options available for users
• Do a deep dive into and produce a write-up of our SignOption type, which currently potentially offers options not covered by rust-bitcoin's Psbt::sign API
• Deprecate all signing and policy related APIs

Related issues and PRs

• feat: implement GetKey for KeyMap and DescriptorSecretKey rust-bitcoin/rust-miniscript#851
• fix(keymap): backport use of correct derivation_path for keys with origin rust-bitcoin/rust-miniscript#867
• 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 finalize_psbt when returning false #73
• docs(signers): what are the signoptions, what are they used for ? #379
• docs(signers): document SignOptions and explain feasibility #379 #434
• Add an example/Make more obvious how to sign a transaction #340
• refactor(wallet)!: remove signers and it's APIs, relying on rust-bitcoin's Psbt::sign instead. #235
• Deprecate wallet-owned signing APIs #505
• feat(wallet): add Wallet::sign_psbt<K> wrapping bitcoin::Psbt::sign #438
• feat(bdk_wallet): add get_keymap() helper method to get the KeyMap for Psbt.sign #334

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 The signer module contains outdated code that is mostly supplanted by recent developments in rust-bitcoin, plus removing a bunch of traits should cut down on code bloat.