We are currently experimenting with using bundles compressed using Zstandard (zstd) rather than gzip to potentially save a significant chunk of the time we spend extracting the bundle. This could lead to a noticeable analysis speedup, particularly for small codebases where fixed costs represent a higher proportion of the total runtime.

This PR is one of the first steps. Specifically:

• We allow users to pass zstd bundles using the tools input to init
• We add support for decompressing these zstd bundles. This works by letting tar auto-detect the compression method, i.e. instead of running tar xz, x for extract, z for gzip, we just run tar x.
• We add telemetry for the compression method used and the version of tar (this informs us whether zstd should be available).
• We download zstd bundles in PR checks to test out this functionality (and hopefully speed up the PR checks!)

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.