Add top-level plots section

[daavoo]

Per iterative/dvc#7477

[dberenbaum]

This may be important for iterative/vscode-dvc#1192.

[skshetry]

We should keep the implementation and this schema always in sync.

What's the format of plots? In the schema, I only see dict (cc @pared, the schema should be stricter): https://github.com/iterative/dvc/blob/512bfc4305a7607f63514dc734663c973ed954b2/dvc/schema.py#L59.

Let me know if you need help here. :)

[pared]

Sorry, @skshetry I didn't add it to my TODO's, I don't expect that help will be needed, adding it rn.