add try-runtime-ci

[brunopgalvao]

closes #76

This PR makes it so try-runtime will run when a migration label is added to a PR.

This is considered a template for a try-runtime CI pipeline. In order for this to be fully functional, an RPC node will need to be setup for the project with flags on it to enable large request/response size.

[brunopgalvao]

This test should fail as the the RPC node is not set up.

[Moliholy]

As a suggestion: I think rather than repeating the same setup in both environments it'd be worth to create a common setup like in trappist to reduce the size and improve readability. In any case this can definitely be a future improvement, so I leave it up to you.

[brunopgalvao]

Took @Moliholy's suggestion and reduced code duplication by creating a custom Github Action action that can be used for both mainnet and devnet. Thanks to @stiiifff for the custom action idea.

[Moliholy]

If we're now using the try-runtime standalone binary we should remove the TryRuntime command, as already done in Trappist. This PR seems like a good place to do so.

[weezy20]

[Suggestion] : Could this PR use an additional document inside docs/ describing the process for try-runtime briefly and setting up / using this github action (like setting up their RPC node) or the try-runtime-cli for downstream projects?