Automatic crate dependency updates

[sethp]

Deterministic builds AND easy dependency maintenance? Why not both?

My ideal flow would be:

1. robot identifies updates (on, say, a weekly schedule?) and opens a PR per (unit) of change
   • implementation: maybe renovate, maybe dependabot, maybe a custom scheduled action workflow that runs cargo update?
   • detail: we can't change esp32c3-hal independently from esp32-hal-common, so those two dependencies ought to be one "unit"
2. each PR gets automatically validated by our CI system (for now, cargo check)
   • detail: we will eventually want to add more thorough checks, but that's out of scope for this issue
   • wrinkle: when there are multiple concurrent units changing, we'll have to re-run
3. when CI succeeds, the PR is automatically merged
4. when CI fails, we hold ourselves to some timeframe to investigate and resolve it (i.e. before the next weekly update?)
   • NB: resolution might be fixing the PR so it's merge-able, "skip this version" + a PR upstream, "skip automatic updates for dependency" + an issue upstream and/or fork

What do you think?