Create a YAML schema file to plan out our changes to our yaml spec

[agjohnson]

I raised in #3878 a good way to start our YAML spec changes would be to write a schema:

I think the next step is to start writing a schema and some actual examples. https://github.com/23andMe/Yamale looks promising.

We should start here so that we can formalize the tree structure and naming conventions, and provide some validation checks.

Work required here might be:

• [x] Start writing a schema file
• [x] Start review early, or separate this work into two PRs
• [x] Add tests around validation of our schema and some example schema fixture files. This won't necessarily replace the need for logic unit tests similar to the build repo now

[stsewd]

I was looking at https://json-schema-everywhere.github.io/yaml, seems like more standard, but there is only a js implementation :(, I'll try to see if the library is mature enough, maybe we can use it doing external calls.

[agjohnson]

I'd be -1 on using a javascript solution here. A python module gives us the ability to quickly turn the schema into unit tests.

[agjohnson]

The underlying PR was just merged. Closing.