Announcement endpoint

[ASpoonPlaysGames]

Similar to /client/mainmenupromos, we should have an endpoint for general northstar announcements that show in the multiplayer lobby on clients.

Current NorthstarMods PR:

R2Northstar/NorthstarMods#699

This PR currently attempts to fetch the data from /client/announcement. It currently expects the response body to be JSON in the following format. All of this is easily able to be changed.

{
    "announcement": "Hello, this is an announcement",
    "announcementVersion": "3"
}

When making a new announcement, announcementVersion must be incremented. Not a lot I can do to get around this, since that's how the vanilla game detects new announcements to display.