github action to release and automagically update version and sha

[lalyos]

This PR adds a new github action workflow:

• if there is a new tag matching vX.Y.Z
• create a new release to store examples.tar.gz of all yaml for the krew plugin
• commits an updated examples.yaml (krew plugin descriptor) with updated version num and shasum

Usage

Once it's merged you just create a new tag (starting with "v") on master, and the workflow does the rest:

• creates a new release matching your tag
• updates the plugins/examples.yaml krew manifest and commits it to master

See it in action

I have tested it on my own fork, see the result:

• See the successful workflow run logs
• the release created bu the action: release v0.0.3
• the commit on master (by github action) fixing url version and shasum: lalyos@ad209ba

Notes

I was trying to use the action: github.com/stefanzweifel/git-auto-commit-action but it seems
this plugin cannot handle pushing to a different branch. It is triggered by the git tag,
so it will be a "detached head" and than couldn't handle switching to the master branch.

see: action error

So I ended up duck-taping a bunch of git command to implement a safe switch to master.