github/ci: New CI framework

[phlax]

README: https://github.com/envoyproxy/envoy/blob/48afe3a868c3a175c393d5feca285cf1345a6c7a/.github/workflows/README.md

config: https://github.com/envoyproxy/envoy/blob/48afe3a868c3a175c393d5feca285cf1345a6c7a/.github/config.yml

Commit Message:
Additional Description:
Risk Level:
Testing:
Docs Changes:
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
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[wbpcode]

Blinded LGTM. Although I had a talk with @phlax offline, it's still almost impossible for me to give a reasonable review to this super large PR.

The annoying point it that we can only verify the CI after we merge it. Anyway, let's do it.

[phlax]

The annoying point it that we can only verify the CI after we merge it

that will always be the case using non pull_request events (i covered this a bit in the README) - im happy to demo how anything works if that is helpful in the staging repo

[phlax]

@wbpcode ive had a go at simplifying the filters in _finish.yml and the upstream ci/request action - hopefully they are a bit cleaner/clearer

[abeyad]

@phlax I scanned the mobile bits of the PR and it looks reasonable to me, but agree with @wbpcode that it's hard to review it thoroughly and understand how it works compared to previous CI. Pretty great if RBE works with this!

[phlax]

it's hard to review it thoroughly and understand how it works compared to previous CI

i added a README to try and cover the headline bits - but we can defo document this better as we go