feat: Added "mark as unread" feature for rooms

[skyegallup]

Description

This PR adds the "unread markers" feature from section 10.7.2 of the Matrix specification. With this feature, users can mark rooms as unread without modifying their read receipts, allowing them to remind themselves to read a room's contents later.

cinny-unread-markers.mp4

As described in the Matrix specification, this is implemented using account data set on the room being marked as (un)read. The account data is updated separately from the user's public or private read receipts.

Partially fixes #2574.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[skyegallup]

I have read the CLA Document and I hereby sign the CLA