Add client endpoint for updating category

Summary of changes

Add endpoint to update category settings via the api. I have tested that the new method adds a new tag group to an existing category against my Discourse instance, see the example code below:

from munch import DefaultMunch

...

category = DefaultMunch.fromDict(discourse_client.category(category_id)["category"])

        if STAFF_ONLY_TAG_GROUP_NAME not in category.allowed_tag_groups:
            category.allowed_tag_groups.append(STAFF_ONLY_TAG_GROUP_NAME)
            discourse_client.update_category(
                category_id, allowed_tag_groups=category.allowed_tag_groups
            )

Checklist

• Changes represent a discrete update
• Commit messages are meaningful and descriptive
• Changeset does not include any extraneous changes unrelated to the discrete change

Add endpoint to fetch latest posts across topics

Added endpoint for looking up latest posts across topics, see Discourse api documentation here:

https://docs.discourse.org/#tag/Posts/operation/listPosts