Add Annotations API

Adds an API to get metric annotations from Mediawiki.
The API uses mediawiki-storage to retrieve the pages.
Also, performs the following validations
on annotations read from mediawiki pages:

* Checks that dates are valid or undefined
* Checks that start date <= end date
* Checks that note is a string

If an annotation is not valid, it is filtered
and the others are returned.

Bug: T78151
Change-Id: If372e9d6065aefc7ad0d6abb49894b18f06b7caf