fix(electrum): `bdk_electrum` coinbase merkle and `populate_with_txids` exploit fixes

[LagginTimes]

Fixes bitcoindevkit/bdk#1698, bitcoindevkit/bdk_wallet#57.

Description

The aim of this PR is to fix a few exploits that were found within bdk_electrum. Most notably:

• Transaction Merkle proof verification does not check the Merkle proof for the coinbase. This enables an attack by which an attacker can fake a deeply-confirmed payment to a BDK wallet. (https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710#merkle-tree-attacks-using-64-bytes-transactions-8)
• An Electrum server can trigger a panic in populate_with_txids by returning a transaction with no output.
• fetch_prev_txout should not try to query the prevouts of coinbase transactions. This will query a transaction to the Electrum server which will return an error and will make the overall sync fail.

Notes to the reviewers

• The "validate coinbase merkle test" commit needs significant review. My initial thoughts were to put MockElectrumClient into bdk_testenv, but because bdk_testenv's electrum-client is re-exported from electrsd, a separate dependency would have to be added just for electrum-client 0.22 support. However, since we are doing in-line tests to access private functions, there is a strong argument for leaving the test structures where they are currently at. Or perhaps there is a better way to specifically test validate_merkle_for_anchor that does not involve creating a MockElectrumClient, similar to what @evanlinjin is doing here: https://github.com/evanlinjin/experiments/blob/main/electrum_c/src/state.rs.

Changelog notice

• fetch_prev_txout no longer queries coinbase transactions.
• Coinbase tx merkle path is also validated during tx merkle path verification.
• populate_with_txids will no longer panic on transactions with no outputs.

Checklists

All Submissions:

• [x] I've signed all my commits
• [x] I followed the contribution guidelines
• [x] I ran cargo fmt and cargo clippy before committing

New Features:

• [x] I've added tests for the new feature
• [x] I've added docs for the new feature

Bugfixes:

• [ ] This pull request breaks the existing API
• [x] I've added tests to reproduce the issue which are now passing
• [x] I'm linking the issue being fixed by this PR

[oleonardolima]

Isn't this one part of 1.0 milestone? I guess it's missing it and not appearing on the project board.

[notmandatory]

For the audit issue bitcoindevkit/bdk#1698 you fixed it with bitcoindevkit/bdk#1756. Can you update this to just be checking Merkle proofs? And if so then I'd say this is a new feature that can be put off to a post 1.0.0 release.

[evanlinjin]

Thank you for working on this. However, I'm not sure how useful this is here, since we aren't even verifying the PoW of the chain in bdk_electrum. bdk_electrum assumes that we can trust the server that they are connected to.

In the future, we should have a version of bdk_electrum which checks PoW and addresses this exploit.