feat(mutations): add custom mutations support

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

This PR introduces new feature allowing to set custom mutation func. This might be useful for:

1. Structural mutations
2. The input contains checksums that have to match the hash of some subset of the data or else the whole thing is invalid, and therefore mutating any of that subset means you need to recompute the checksums.
3. Small random changes to the input buffer make it invalid.

To use custom mutation, just export mutate function.

See attached example