Add release-plz

[andrewphilipsmith]

This PR establishes a workflow for publishing to crates.io using release-plz.

Notes:

• Untested so far
• Aligns the version number for all three crates.
• Minor tweaks to the package metadata to satisfy crates.io's requirements.
• Uses a GitHub App for GitHub authentication (See docs)

[andrewphilipsmith]

Remove the branch name add-release-plz before merging this PR. The only remaining branch name should be main.

[andrewphilipsmith]

The important changes in this PR are mostly the authentication settings. In theory, it can only be reviewed by someone who has Owner permissions on the GH Org (in practice, this is not a problem as most popgetter contributors have this access). To review and complete this PR:

• A reviewer should check that they can access the popgetter-release-bot App settings here:
  https://github.com/organizations/Urban-Analytics-Technology-Platform/settings/apps/popgetter-release-bot
  This gives the App permission to make modifications to the code and create new PRs. (See chore: release v0.2.0 #98 for an example). They should agree with the current settings and permissions for the App, whilst also checking that they have permissions to edit the App.

• Confirm they have shared ownership of the packages on crates.io. The other owners must be added manually for each package.

  • Create a list of crates.io usernames to share with.
  • Ensure that each of those usernames has a verified email address

• Confirm that we no longer require the repo secret CRATES_TOKEN and remove it from the repo settings.

• Remove this line in the workflow file:
  https://github.com/Urban-Analytics-Technology-Platform/popgetter/pull/97/files#r1913030306

[andrewphilipsmith]

Have added @stuartlynn and @sgreenbury to the owners on crates.io.