grizzly/actions/setup GitHub action

[K-Phoen]

This PR introduces a GitHub action that can be used to setup Grizzly in a job.

I'm thinking that having two github actions could be nice:

• one to setup the binary and nothing else, giving complete freedom to the user
• a second to make "apply" actions easier (it could install grizzly, help configuring it and wrap the apply command)

Both these actions would make it easier to build automation on top of grizzly, in GitHub actions.

Open questions:

• where should these actions be published? Having them in this repository is convenient, but makes versioning them much harder

[julienduchesne]

I feel like we could use a go install composite action and avoid a lot of javascript here

Also, do we have somewhere we want to use this or is this for future use?

[K-Phoen]

I feel like we could use a go install composite action and avoid a lot of javascript here

Running go install in a composite action would require us (or the caller) to setup go first, and then we would download Grizzly's dependencies and re-compile the binary every time this action is called.

I wanted to ensure fast runtimes by downloading pre-compiled binaries instead, and taking advantage of GitHub action's "tools cache". That last part is why this action is written in Javascript.

Unfortunately, I also just discovered that this cache isn't persistent across workflow runs :( (see actions/toolkit#58)
That probably means that a composite option that downloads the binary from our releases is a valid option again.

Also, do we have somewhere we want to use this or is this for future use?

Nothing specific in mind, this PR is just me fooling around 😅
I'm still guessing that a "grizzly/setup" action could be a useful thing to offer, for everyone running grizzly in their github pipelines.

Even though it could be made slightly more useful by providing ways to setup Grizzly's configuration/contexts. Something like:

- uses: grafana/grizzly/actions/setup@main
  with:
    version: 'v0.4.0'
    context:
      grafana.url: https://my-instance.grafana.net
      grafana.token: ${{ secrets.GRAFANA_TOKEN }}

- run: grr diff ./resources

Admittedly, this action isn't absolutely necessary to use grizzly in a github action. I just feel like we could provide it to remove some of the boilerplate necessary to set it up 🤷

WDYT?

[BrendanCoughlan5]

+1 please land this, i'd love it :)

[BrendanCoughlan5]

plz land this xD

[mhixon4479]

+1 to landing this :)

[K-Phoen]

We released this GitHub action in its own repository: https://github.com/grafana/setup-grizzly