feat: Add feature to enable dynamic ec2 config via workflow labels

[edersonbrilhante]

Summary

This PR resumes and completes the work started in #4529.

It also allows to use any other dynamic labels with prefix ghr-. Giving support for unique labels per job or per group of jobs

It ensures that EC2-specific config can be defined via run-ons

How to test:

Use your regular labels, and add ghr-ec2-instance-type and ghr-ec2-image-id

run-ons:
  - <regular-labels>
  - ghr-ec2-instance-type:<different-instance-type>
  - ghr-ec2-image-id:<different-ami-id>

In this case:

• The runner is resolved from <regular-labels>
• The EC2 instance type and AMI ID are taken exactly from the provided labels

[npalm]

@edersonbrilhante great to see this PR.

[andrecastro]

This is a really interesting feature!

Just one suggestion from my side: would it be possible to support a whitelist of allowed instance types?

Also, it could be really powerful to have some kind of feature-flag / policy control over which parts of the configuration are allowed to be dynamic. For example, in my org we don’t want developers to be able to select arbitrary AMIs (only a pre-approved set), but it would be awesome to still allow them to choose the instance type for workflow jobs, as long as it’s constrained to an allowed list.

Maybe the "feature flag" is not even necessary as long as we could define the "allowed values" for each configuration, with this we could list only the pre-approved AMIs.

[edersonbrilhante]

@andrecastro I liked and makes a lot of sense to me. I just need more time to think about the implementation. And tbis PR is already big enough XD. I could create a following for adding this restricted values feature