Use dialect identifier from vega-lite-schema to set jsonschema validator

[mattijn]

As was pointed out in #2771 (comment), and summarised here:

• vega specifies the $schema property in the schema (see for example L3 here), which is a "dialect identifier", it basically is a URI that identifies which version of the JSON Schema spec the schema is written for -- an example of one is https://json-schema.org/draft-07/schema which is the dialect ID for draft 7 of the spec
• we can then use jsonschema.validators.validator_for to get the right validator class given the schema, which will return you Draft7Validator if the schema use draft 7, but if/when Vega-lite repo upgrade to draft2020 say, then you'd get Draft202012Validator instead and things will work properly still.

Currently (or at least after PR #2771 is merged) the validator version is defined as a constant (draft 7). Altair currently uses this validator for the schema definitions for Vega-lite v3/v4/v5 (and that is still fine as they all use the draft 7 schema). So in my understanding the required change to get this working properly is to make it dynamic and set and define the validator on a version by version basis.

[binste]

Thanks for the summary! I'll look into this.