Add support for release-plz

[antiguru]

This adds initial support for release-plz and configures it to only produce a changelog for the timely crate.

It adds the following files:

• .github/dependabot.yml: Check that we're running recent versions of github actions. It doesn't check for crate updates, that'd add a lot of noise.
• .github/workflows/release-plz.yml: The action to enable release-plz. Pretty much the most basic variant I can imagine.
• release-plz.toml: Configure release-plz to only produce changelogs for the timely crate. We could think about disabling the changelog entirely. (While it maintains a release PR, it's easy to manually edit the changelog before merging.)

[frankmcsherry]

Fascinating! Let's try it!