Migrate transaction building from `TxBuilder` to `Wallet::create_psbt`

[vadim-anfv]

The plan is to transition bdk_wallet to depreciate TxBuilder and use bdk_tx via Wallet::create_psbt (see bitcoindevkit/bdk_wallet#516). So I'd suggest adding the check to bdk-tx and updating bdk-cli to enable the Wallet::create_psbt experimental feature and using it. But that all will need to be done in new PRs.

Originally posted by @notmandatory in #326 (comment)

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This issue is about moving bdk-cli to Wallet::create_psbt from bdk_wallet 3.2.0. The tx size check discussed in the same thread is out of scope here.

bdk-cli is on bdk_wallet 3.1.0 and builds every transaction with TxBuilder. Wallet::create_psbt is available in 3.2.0, behind the bdk-tx feature and the --cfg bdk_wallet_unstable rustflag.

Call sites to migrate:

• CreateTxCommand in src/handlers/offline.rs
• CreateSpTxCommand in src/handlers/offline.rs
• BumpFeeCommand in src/handlers/offline.rs, the only one using build_fee_bump
• CreateDnsTxCommand in src/handlers/dns/mod.rs
• PayjoinManager::send_payjoin in src/handlers/payjoin/mod.rs

Notes:

• Wallet::replace_by_fee is the counterpart for bump_fee.
• The bump also hits deprecations outside tx building, which -D warnings in CI turns into blockers: Wallet::sign, Wallet::get_signers, Wallet::policies, TxBuilder::policy_path and LoadParams::extract_keys.
• The bump alone already breaks just pre-push: every test that signs a PSBT fails with Signer error: Missing non-witness UTXO, to be sorted out as part of this work.