Run backend e2e tests with given gateway docker tag when provided in PR body, add dev docs

[dandelany]

___REQUIRES_GATEWAY_PR___="116"

Description

Closes #1538

For reference see NASA-AMMOS/aerie-ui#1439

This PR contains two things:

• An update to GH workflows to allow aerie backend PRs to specify an aerie-gateway PR/image to use for e2e tests - since, as noted in this comment from @Mythicaeda, the backend e2e tests may rely on changes in Gateway. This is an exact copy of the approach from Run UI e2e tests with given aerie/gateway docker tags when provided in PR body aerie-ui#1439 applied to the aerie backend e2e tests
• Updates to the CONTRIBUTING.md developer docs to explain how to use this process. Also added some notes on Required Checks in general since they weren't mentioned before.

Verification

I tested this PR both with and without the REQUIRES_GATEWAY_PR tag at the top, and verified that when it is present, the e2e test action correctly pulls and runs the gateway image tagged as pr-116 (shown in the workflow run logs)

[Mythicaeda]

I'd recommend using ___REQUIRES_GATEWAY_PR___="119" instead, as that contains no behavior changes to the Gateway code.

Ah, I see why you're using 116, since it's already publishing an image.