cleanup: Use generics to simplify code for parse_config and related builtins

[jalseth]

No description provided.

[boranx]

lgtm

Thanks for the refactor and switching to the generics
What about refactoring the other places as well? (like engine or parser)

[jalseth]

I think they could be used in parser, as a lot of logic that package is:

1. Unmarshal []byte from format XYZ to struct
2. Marshal struct to JSON
3. Unmarshal from JSON to map[string]any for use with OPA

This could be written generically as Parse[T any](data []byte, func(data []byte, temp T) error) (map[string]any, error) which would accept the data and an config-specific unmarshal function (step 1) and then steps 2 and 3 are the same regardless of config.

[jalseth]

Opened #861 to discuss this further