feat: add completion for additionalProperties propertyName

[p-spacek]

What does this PR do?

Add the possibility to have code completion on properties of 'anonymous/unknown' object.
Example:
typescript:

object: Record<string,string>

will produce approximately this schema:

"object": {
  "type": "object",
  "additionalProperties": {
    "type": "string"
  }
}

then in the yaml it could be complicated for users to guest what can be inside the object

object:
  # no codecompletion result, no possibility for documentation

But jsonschema standard allows propertyNames element to extend properties json schema doc

So by this PR, this schema:

{
  "type": "object",
  "additionalProperties": true,
  "propertyNames": {
    "title": "key of obj",
    "description": "desc",
    "markdownDescription": "*md description*"
  }
}

will produce this result:

Additional notes

Note that defaultSnippets can do almost the same:

{
  "type": "object",
  "properties": {
    "prop1": {
      "type": "string"
    }
  },
  "additionalProperties": true,
  "propertyNames": {
    "title": "key of obj",
    "description": "desc",
    "markdownDescription": "*md description*"
  },
  "defaultSnippets": [
    {
      "label": "snippet property",
      "body": "${1:property}",
      "suggestionKind": 7,
      "markdownDescription": "*md description*"
    }
  ]
}

But with propertyNames implementation it will be possible to add some documentation info about pattern, min, max in the future.

What issues does this PR fix or reference?

no ref

Is it tested? How?

add UT

[coveralls]

Coverage increased (+0.04%) to 82.435% when pulling 591b7b9 on p-spacek:feat/add-completion-for-additionalProperties-propertyNames into d0e26bc on redhat-developer:main.

[msivasubramaniaan]

LGTM

Thanks for your contribution