Direct Message [Action Sheet]: add Action sheet for a DM conversation

[Abhisheksainii]

Fixes #1272

In this PR, I added the option to open an action sheet for a DM that does 2 things :

1. Mark all messages as read for that DM.
2. Gives the ability to view the members in a group DM.

The action sheet can be opened through 4 different locations:

1. A DM row in the Inbox
2. A recipient header in the message list
3. The area of the message-list app bar that identifies the narrow
4. The "Direct messages" page

The steps I followed to implement the above:

1. Constructed a DM action sheet class by extending it from ActionSheet abstract class.
2. Built DM users page (Figma design) with the ability to search for the users in the list.
3. made use of enums to highlight the sheet option, i.e, either mark messages are read or view group DM users.

Attaching screenshots for reference.

WhatsApp.Video.2025-02-06.at.00.51.08.mp4

[chrisbobbe]

Thanks @Abhisheksainii for your work on this!

Given that this issue is for the M6 Post-launch milestone, and that this PR doesn't yet have tests so still has significant work ahead before we can merge it, let's put it on hold for now until the launch is ready — see announcement.