type annotations for metainfo

Hello :) Very simple little PR, took me awhile to get a sense of the structure of the metainfo object, so thought i might as well contribute some type annotations for it. `typing_extensions` dep is for the `Required`/`NotRequired`/`total` syntax which is only in >3.11, that's necessary to model the "anything can be in there but these are the keys that are usually in there" structure of that dict.