Frame out the env spec cep

[ericdill]

discussed with @mariusvniekerk, @AlbertDeFusco , @jezdez and @jaimergp.

opening this PR as a draft PR for further conversation. it's not really ready for full review yet.

there's a notebook that validates the env yamls that are part of this PR. those env yamls (and the validation code) should probably move over to the conda specs repo once this is finished and accepted.

[AlbertDeFusco]

Should this CEP also mention which of the keys are strictly required and which are optional? I've found that only dependencies is required and the rest can be left off.

[jaimergp]

For reference(s), this epic lists a few issues where similar conversations happened: https://github.com/conda/conda/issues/11341

[jaimergp]

Btw, thanks to @AlbertDeFusco I added a pydantic model to the ongoing effort on conda/schemas:

https://github.com/jaimergp/schemas/blob/revamp/conda_models/environment_file.py