Add support for Grafana Cloud Fleet Management

[johannaojeling]

Adds support for managing Grafana Cloud Fleet Management resources.

Changes:

• Adds 2 new fields to the provider
  • fleet_management_auth
  • fleet_management_url
• Adds 4 new fields to the Cloud stack resource and data source
  • fleet_management_name
  • fleet_management_status
  • fleet_management_url
  • fleet_management_user_id
• Introduces a new Fleet Management Collector resource
• Introduces a new Fleet Management Pipeline resource

[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.

[tpaschalis]

I don't have much experience around Terraform and providers, but after scanning through these a couple of times and going through a demo, this LGTM, but I'd appreciate a review from someone with more experience. Let's make sure that we're marking resources with the correct stability labels for the future.

Nice work!

[Duologic]

I had a look at the more common parts (client, examples, stuff in pkg/), LGTM!

I didn't dig too much into the resources themselves, I think its better for the team behind fleet management to have a look at those and check if they are correct.

[johannaojeling]

Blocked by #2024 to fix the failing unit tests / docs check