Add an `envoyconf` linting/validation tool

## description

If we create `jsonschema` for validating envoy `yaml`/`json` config files (see #13078) we can potentially re-use the schema for a `cli` linter (and/or libs for python/js etc).

This could eg be used to:

- validate snippets in docs etc (see #8837 and #13231)
- validate deployment config for end users locally or in eg gitops-style CI
- maybe also for creating/manipulating config programatically

## refs

- #13078 (vscode)
- #8837 (config validation)
- #13231 (move snippets to rst includes)
- #13167 (automate dumping of typed extensions)
- https://github.com/chrusty/protoc-gen-jsonschema
- https://github.com/cwacek/python-jsonschema-objects
- https://github.com/envoyproxy/envoy/tree/master/test/tools
- https://www.envoyproxy.io/docs/envoy/latest/install/tools/tools
- https://github.com/envoyproxy/envoy/issues?q=is%3Aopen+is%3Aissue+label%3Aarea%2Froute_check_tool#
- https://www.schemastore.org/json/
- https://envoylint.com/
- https://github.com/ysawa0/envoylint