Create JSON schema for hubmeta

[annakrystalli]

Wasn't sure where the best place for this issue is. I decided here but we can reference it in other repos to keep track of it

The purpose of the schema is two-fold:

1. Act as documentation of expectations of vallid hubmeta.json.
2. Be used to validate json hubmeta files against.

PROs

• simple one step validation which can be performed using many languages/tools. This means validation is not encode in language specific code.
• human readability of file makes standards inspectable by users

CONs (within the hubUtils) context

• package jsonvalidate which can be used to perform validations depends on package V8 and underlying V8 javascript and webassembly engine. Since 2020, it's much easier to install V8 (work out of the box with install.packages("V8") on maOS & Windows) it does need separate installation of the V8 C++ engine on Linux. This used to be problematic on systems e.g. without sudo rights but an alternative option to automatically download a static build of libv8 during package installation is now available.
• While it appears json schema can be used to validate yaml, I can't find a tool for it in R. We can always however convert yaml to json prior to validating.

Initial resources

• jsonvalidate R package
• JSON Schema: A Media Type for Describing JSON Documents
• Understanding JSON Schema ebook.
• Online JSON to schema converter. Useful for getting started.
• Frictionless data: Validating tabular data with a specialised JSON schema
• Stencila use of JSON schema for validating notebook elements. Useful example of generating schema for custom objects.