Add intended release channel

[iowillhoit]

What does this PR do?

Allows you to designate what channel this PR is intended for. This channel will get picked up by GHA to support: latest-rc, latest, nightly, and prereleases

What issues does this PR fix or reference?

@W-12599588@

[iowillhoit]

EDIT: I went ahead and made this change. I think it is less confusing

One potential change... Right now it is required that the release channel is one of latest, latest-rc, or nightly. I did this because we want to require "dev releases" to be prereleases. I could change the validation logic a bit to say:

if ( release-channel is not one of [latest, latest-rc, nightly] && !prerelease) {
    throw Error("prerelease must be set to true if release-channel is not one of [...]")
}

This could potentially remove some confusion around the exactlyOne: ['release-channel', 'prerelease']. The release-channel is set on a single flag, but we enforce prerelease on non-standard channels