Add minimal dtool dataset metadata schema

[jotelha]

I would like to create a universal JSON schema for validating metadata attached to dtool datasets (see articles https://peerj.com/articles/6562/, https://doi.org/10.1371/journal.pone.0306100, and dserver demonstrator at https://demo.dtool.dev).

I have no experience with creating schema, but I believe these changes should be a minimal starting point.

[github-actions]

Thanks for the PR!

This section of the codebase is owned by @madskristensen and @hyperupcall - if they write a comment saying "LGTM" then it will be merged.

[jotelha]

My apologies for cluttering this PR, I opened it a little early.

I have renamed the schema files to a versioned scheme.

Now, I have somehow messed up and cannot get rid of this error

$ node ./cli.js check 
===== VALIDATE PRECONDITIONS =====
---
>> Failed to find a schema file at "src/schemas/json/dtool-dataset-metadata.json"
---
Error
    at onTestDir (file:///mnt/dat2/git/dtool/schemastore/cli.js:839:29)
    at async Promise.all (index 0)
    at async assertFileSystemIsValid (file:///mnt/dat2/git/dtool/schemastore/cli.js:831:5)
    at async Object.taskCheck (file:///mnt/dat2/git/dtool/schemastore/cli.js:564:3)
    at async file:///mnt/dat2/git/dtool/schemastore/cli.js:1560:5

anymore, which I do not quite understand, as nothing in the tree points to the initial dtool-dataset-metadata.json without version suffix anymore.

Apologies again, some help would be much appreciated. Thank you!