zimcheck: check known/expected metadata mimetype

[benoit74]

As mentioned by @rgaudin in https://github.com/openzim/mwoffliner/issues/2470, it seems that all known/expected metadata (but illustrations) should have text/plain or text/plain;charset=UTF-8 mimetype.

It looks like zimcheck is not checking this at all.

Probably worth to add. Not sure if it should be an error or a warning.