github action to run bids-validator on bids-examples

[yarikoptic]

inspired by

• Make -s|--schema take path or add new --schema-file option to point to schema.json file #1930

I think we should make it very easy for PRs against bids-specification, which change schema, to run validator using modified schema against target (or all) bids-examples. Originally I thought it should be just a matter of documentation:

• Rework CONTRIBUTING:BEPs section for schema-driven approach #1828

but I think we just need an action so we could just have

- uses: bids-standard/bids-validator@master
  with:
     bids-examples-glob: *
     schema-version: inplace

by default in bids-specification and make it active for any change to src/schema.

And then BEP PRs would gain possibility to point to a corresponding branch in bids-examples, e.g.

- uses: bids-standard/bids-validator@master
  with:
     bids-examples-glob: *
     bids-examples-fork: yarikoptic/bids-examples
     bids-examples-branch: bep032
     schema-version: inplace

this would make it super easy to ensure that changes to schema are validated properly and hide away all needed intricacies of setting up/running bids-validator (#1930).