Enable loading of customTags from inside the JSON schemas themselves

JSON schemas can contain addition information that is not part of the schema itself, this means that they can also expose which customTags are expected to be found on a specific document.

This could be of great use because each schema published is the likely person known which tags their projects support. It would also mean that we avoid avoid the need to force users to add all kind of customTags to their user settings.json. Also adding global such customTags is not the best practice because it would prevent us from detecting if a tag unsupported by one product/file-type is added.

I propose looking for something like:

{
    "$schema": "https://json-schema.org/draft/2019-09/schema",
    "properties": {
        "customTags": ["!encrypted/pkcs1-oaep sequence"]
    },

I verified and that schema is passing validation.

The issue names no files or tests. Start by tracing how JSON schemas are loaded and how customTags are currently read from user settings, then determine how schema metadata should be recognized and validated. Done means supported customTags from a schema are applied to its documents without requiring equivalent global settings.