Add release-plz

[berkus]

Automatic release/publish pipeline.

You'll need to do two things to set it up: one mandatory and one optional.

Mandatory step:

1. generate a publish token on crates.io (preferably scoped to avra-rs, needs only "publish new packages" permission)
2. Paste it into an env var called CARGO_REGISTRY_TOKEN in github:

Optional step:

1. Add branch protection on the master branch, forbid direct commits.

If you don't - release-plz will trigger a release immediately after a merge to master and will commit a new release into master directly.
With protection, release-plz will create a PR with all the changes it wants to do and only after this PR is merged it will make a tag and a release, so this lets review changes before releasing.

(First release might need to be patched manually a bit to synchronize changelog to what release-plz expects.)