Use AJV to validate referenced schemas

[shin19991207]

What does this PR do?

Meta-validate the referenced schema of a YAML file against the correct meta-schema based on its $schema value.

This change support meta-validation for:

• draft-04
• draft-07
• draft 2019-09
• draft 2020-12

What issues does this PR fix or reference?

This PR replaces the hyperjump-based schema meta-validation approach in #1115 with AJV (we already depend on AJV in this project and AJV can validate JSON Schema drafts).

This improves schema meta-validation and is complementary to (but not a prerequisite for) the broader work tracked in #856 to support newer-draft instance validation.

Is it tested? How?

Added automated tests covering draft-specific meta-validation failures for each draft version supported.

Manually verified using the following JSON schemas (create a YAML file containing just the number 1 and point it to each schema):

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "number",
  "minimum": 5,
  "exclusiveMinimum": 5
}

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "number",
  "minimum": 5,
  "exclusiveMinimum": true
}

{
  "$schema": "https://json-schema.org/draft/2019-09/schema",
  "$defs": {
    "foo": {
      "type": "object",
      "properties": {
        "bar": { "type": ["foo", "bar"] }
      }
    }
  }
}

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "number",
  "prefixItems": "foo"
}

[coveralls]

coverage: 83.677% (+0.02%) from 83.653%
when pulling 511c5c6 on shin19991207:chang-patch-1
into a38caf0 on redhat-developer:main.

[datho7561]

Not strictly needed for this change, but I'm okay with adding it now, since we'll need to do it eventually.

[datho7561]

Looks good. Thanks, Morgan!