feat: add initial fuzz testing

[oleonardolima]

Description

It's an initial work towards the goal of #61, it adds a new fuzz crate to the project which is meant to be used to add new fuzz target in order to increase the fuzz coverage.

It's built on top of and with inspiration from previous Antoine's work through the most recent BDK Audit.

So far, this PR:

• creates a new fuzz crate with cargo fuzz relying on LibFuzzer engine.
• adds an initial fuzz target, which builds and applies an Update to Wallet.
• adds both initial fuzzed_data_provider and fuzz_utils with common used fns to build BDK's-specific types from fuzzed bytes.

Still to be done by this PR:

• write-up the README.md with instructions for other users.
• add helpful bash scripts, to build the environment, and run the existing fuzz targets.
• add a new CI step, to briefly run the existing fuzz targets at a certain schedule.
• add remaining: (i) persist ; (ii) tx creation fuzz targets (by Antoine's existing target).
• add other fuzz targets.

Notes to the reviewers

Are there any other BDK usage scenarios you'd like to see covered by a fuzz target ? Let's discuss it either on the issue or under this PR comments.

Changelog notice

TBD

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

[coveralls]

Pull Request Test Coverage Report for Build 16149185029

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 94 unchanged lines in 7 files lost coverage.
• Overall coverage decreased (-0.8%) to 84.733%

---

Files with Coverage Reduction	New Missed Lines	%
wallet/src/descriptor/dsl.rs	1	95.34%
wallet/src/wallet/changeset.rs	2	85.0%
wallet/src/descriptor/policy.rs	3	79.07%
wallet/src/wallet/coin_selection.rs	3	98.48%
wallet/src/descriptor/template.rs	4	98.04%
wallet/src/wallet/tx_builder.rs	39	93.35%
wallet/src/wallet/mod.rs	42	81.03%

Totals
Change from base Build 15719064446:	-0.8%
Covered Lines:	6577
Relevant Lines:	7762

---

💛 - Coveralls

[evanlinjin]

Could you include the rationale for having a separate crate for fuzzing? Is it because we need to use nightly cargo?

[TheBlueMatt]

Rather than requiring use of libFuzzer, it might make sense to build a generic fuzz wrapper, possibly based on the LDK fuzz infra at https://github.com/lightningdevkit/rust-lightning/tree/main/fuzz or the rust-bitcoin fuzz infra. libFuzzer is great and one of the more actively maintained fuzz harnesses, but honggfuzz is in many cases easier to run, so we've found it quite useful to offer both (plus AFL, for those who want that).