feat(capi): consider to use CAPI: 2 instead of CAPI=2:

[tymonx]

Context: #792 and #800

Using the CAPI=2: directive makes loading the correct JSON schema for validation harder not easier. Because tools will require to iterate over YAML keys like CAPI=2, CAPI=3, CAPI=4, .... The proposition is to replace the CAPI=2: with more agnostic CAPI: <version>, make it part of JSON schema but still keep it backward compatible. Additionally, the CAPI: could be optional. This will make FuseSoC core files closer to any existing YAML formats.

JSON schema proposition:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "CAPI2",
  "description": "Core API",
  "type": "object",
  "properties": {
    "CAPI=2": {
      "description": "Short for 'Core API version 2'. The schema used in core files.",
      "type": "null"
    },
    "CAPI": {
      "description": "Short for 'Core API' version. The schema used in core files.",
      "type": "integer",
      "minimum": 2,
      "maximum": 2
    }
  },
  "oneOf": [
    "CAPI=2",
    "CAPI"
  ]
}

This will simplify loading and validating core files to few lines:

SCHEMAS: dict[int, str] = {
    2: capi2_schema,
}

@functools.cache
def schema_validator(version: int = 2) -> Callable[[object], None]:
    """Get function to validate data againts JSON schema. Used schemas are cached for performance."""
    return fastjsonschema.compile(SCHEMAS[version])

def parse(file: Path | str) -> dict[str, Any]:
    with open(file) as fp:
        data = yaml.load(fp)

    schema_validator(data.get("CAPI", 2))(data)

    return data

In current implementation, compiled schema is not cached.