feat: make plugin version optional - installs latest version

[s4kh]

Fixes #2312
Tested with dev instance on local using locally built provider.
resource.tf

terraform {
  required_providers {
    grafana = {
      source = "registry.terraform.io/grafana/grafana"
    }
  }
}
provider "grafana" {
  cloud_api_url             = "https://grafana-dev.com/api"
  cloud_access_policy_token = "dev token"
}
resource "grafana_cloud_plugin_installation" "test" {
  stack_slug = "skh3"
  slug       = "axiomhq-axiom-datasource"
}

[github-actions]

In order to lower resource usage and have a faster runtime, PRs will not run Cloud tests automatically.
To do so, a Grafana Labs employee must trigger the cloud acceptance tests workflow manually.

[s4kh]

cloud acceptance tests

[DanCech]

I kind of wonder whether it would make sense to support latest as an explicit version vs making the attribute optional, then we could potentially check against the plugin catalog in Read whether the installed version is the latest, if so report latest so there's no drift but if not then report the actual installed version to trigger an update.

[s4kh]

@DanCech yeah.
But do we need to auto update the plugin?
There is a following scenario:

1. Plugin is installed without the version specified.
   a. We don't return installed version in the read context(current state) function - if we do, terraform will try to delete and create again due to installed version != "" (declared in the resource.tf) loop happens
2. New plugin version is released
3. We run terraform plan
   a. It will show nothing to apply - because we don't show version in the readContext function because of 1a.
4. What do we want in this scenario?

If we prefer stability and don't want to auto update, current implementation is fine. But in order to install the latest after the release, user either has to define the version number in the resource.tf

[DanCech]

yeah, I'm thinking that if the user asks for the latest we should keep it up to date, and if they want a specific version they should specify it. Hence the idea of using an explicit version: latest in the spec and having read return version: latest if the installed version is the latest, but version: <installed version> if it's not the latest, which would trigger it to update to latest.

[DanCech]

vs asking for latest and getting whatever it happens to be at install time but then never updating.

[s4kh]

defaults to latest

[DanCech]

LGTM, I don't have particular religion about whether the version param is optional with latest as the default or required.