Investigate using `taskcluster` crate ecosystem

[ErichDonGubler]

Suggested in #developers:mozilla.org on Matrix by @ahal (link).

• https://docs.rs/taskcluster/
• https://docs.rs/taskcluster-download/

[ErichDonGubler]

Based on knowing nothing about the crates in question:

Pros

• Potentially code I don't have to maintain, operate at a higher level.
• Maybe more performant download operations, if that's been engineered better than my dinky little pool of things

Cons

• Longer compiles, since we're compiling something more comprehensive than just reqwest?
  • Might otherwise bring in baggage for code that's already working.
  • If there are problems, yet another layer to go through.
  • Licensing?

The above doesn't seem super compelling for investigating immediately, so I'm happy to leave it in treeherder-dl's issue queue for now. It'd sure be cool if we could net s'more perf., tho!