Adds envoy-versions.json and its schema

[codefromthecrypt]

Follow-up from https://github.com/tetratelabs/getenvoy/pull/281

This is not fully automated yet, intentionally

[netlify[bot]]

:x: Deploy Preview for archive-envoy failed.

:hammer: Explore the source changes: 6040d49a4896ac8bd6f324342c45e8fd0b8103ed

:mag: Inspect the deploy log: https://app.netlify.com/sites/archive-envoy/deploys/60caf42a7087db8fef95c010

[codefromthecrypt]

deploy preview doesn't seem top redirect properly.. will look into it tomorrow

[codefromthecrypt]

ok works now

$ export ENVOY_VERSIONS_URL=https://deploy-preview-2--archive-envoy.netlify.app/envoy-versions.json
$ getenvoy versions -a

VERSION RELEASE_DATE
1.18.3  2021-05-11
1.17.3  2021-05-11
1.16.4  2021-05-11
1.15.5  2021-05-11
1.14.7  2021-04-15
1.13.7  2020-12-07
$ ENVOY_VERSION=1.13.7 getenvoy run --version
downloading https://archive.tetratelabs.io/envoy/v1.13.7/envoy-v1.13.7-darwin-amd64.tar.xz
starting: /Users/adrian/.getenvoy/versions/1.13.7/bin/envoy --version
working directory: /Users/adrian/.getenvoy/runs/1623843730656111000

/Users/adrian/.getenvoy/versions/1.13.7/bin/envoy  version: 56d148b94ef25a026d8d31790695d478ad53eb94/1.13.7/Clean/RELEASE/BoringSSL

[codefromthecrypt]

will rework this a little so that the envoy-versions.json file is checked on PR

[codefromthecrypt]

I'm going to merge this because it isn't released yet. there's some cache I can't clear and I think it will work mostly :D

[mathetake]