chore(release): generate changelogs with git-cliff

[hawkw]

We maintain separate changelogs for the tokio-console, console-api,
and console-subscriber crates.

Currently, changelogs are generated using the clog CLI tool.
However, clog does not support generating separate changelogs for
different subdirectories of a repo, so the current release process for a
given crate requires generating a changelog for the whole repo since the
last tag, and then manually editing it to remove changes to other
crates. This is a pain.

The git-cliff changelog-generating tool supports generating a
changelog that only includes commits that modified specific paths in the
repo. This means it can be used to generate separate changelogs for each
crate automatically, unlike clog. It's also much more configurable,
and we can use it to generate changelogs that have somewhat nicer
formatting (IMO).

This branch changes changelog generation from using clog to use
git-cliff, and regenerates all the existing changelogs. The included
bin/update-changelog.sh shell script updates the changelog for a given
crate to the provided tag. This will make the release process much less
complicated.

A subsequent PR will add a shell script that handles updating crate
versions, generating the changelog, and publishing the crate and tag, so
that a release can be published by running a single command.