add try-runtime github workflow

[brunopgalvao]

In alignment with #197

• Check that the try-runtime feature builds
  • We do not need this check on every PR. If anything the try-runtime feature will fail when it runs with the migration label
• Run try-runtime on PRs marked with a migration label

[brunopgalvao]

So I need feedback on which way to move forward. Currently, with this PR, we have it configured so that when we mark a PR with the migration label, it will kick off try-runtime for that PR and test the migration - this is currently how the FRAME team does it. However, they are moving away from this, to a more secure approach by running try-runtime on every commit of every PR regardless of the migration label:

• ci: always run runtime migration checks polkadot#7422

Is this something that we would like to do as well?

From my point of view, having try-runtime kick off on the migration label is good enough for our purposes. If we want an added layer of reassurance we could additionally run try-runtime when we create a "draft" for the release (it would serve as a catch-all).

[brunopgalvao]

We also want this workflow to run on subsequent pushes.