Add feature gate prompt and backport label automation

[jstarry]

Problem

Lack of feature gate process and visibility

Summary of Changes

• Adds an optional prompt for "Feature Gate Details" when creating pull requests which requests devs to describe feature gate changes and conditions as well as add the new "feature-gate" label to their PR for visibility
• Adds automation for copying "feature-gate" labels to backport PR's
• Adds automation for applying the "feature-gate" label to PR's that change the sdk/src/feature_set.rs file

Fixes #

[t-nelson]

I wonder if we can automate adding the feature-gate label by scanning changes to sdk/src/feature_set.rs?

[jstarry]

I wonder if we can automate adding the feature-gate label by scanning changes to sdk/src/feature_set.rs?

Ah great call, added that as well. I think it's still useful to automate copying the label for backports because some feature gate related changes don't change that file.

[t-nelson]

I think we'd only want to do it on solana_sdk::declare_id additions though, right? otherwise we tag removals other other irrelevant updates too. mergify may not be the tool for this job

[jstarry]

Oh true, well then I think the pull request template prompt should be sufficient for devs to develop the habit of annotating PRs with the label if they add feature gate changes. I'll revert the automation for adding the label

[t-nelson]

lmao. half of folks don't even fill it out until after creating the PR. I get so many empty template emails.

I mean, I'm fine with the attempt. I'm just not optimistic it's going to be adhered to in any reliable manner

[jstarry]

Yeah it's a best effort thing to improve visibility. I'll at least find it useful for searching for my own feature-gate PR's. Good to go then?

[t-nelson]

yeah sure. I barely know how mergify works