feat: add flag for limiting the diff depth of snapshots

[lemmih]

Summary of changes

Changes introduced in this pull request:

• Add --diff-depth flag to forest-cli archive export. All values reachable in the diff depth will not be included in diff snapshots.
• Fix type of diff from ChainEpochDelta to ChainEpoch.

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

[lemmih]

I am not sure how to test this code.