Set the HTTP user agent

[gnprice]

Currently when we make an HTTP request, the User-Agent header we send is just whatever the default is that Dart's HTTP implementation gives us. This means e.g. that log lines on the server look like this, saying "via Dart":

2023-11-21 22:32:07.353 INFO [zr] 192.168.0.188   POST    200 113ms (db: 35ms/41q) /api/v1/register (11@zulip via Dart)

Instead we should set a user-agent string that identifies this app. Ideally it should include the app's version number.

There are at least two different kinds of HTTP requests the app makes, both of which should have the user agent set:

• Zulip API calls, which go through ApiConnection.send;
• fetching images, including avatars, image emoji, and images included in messages, all of which go through RealmContentNetworkImage.

It's likely those will need to be handled separately.

[gnprice]

The value should probably say "ZulipMobile" somewhere, because apparently (#453 (comment)) for send-message requests that will activate the server's heuristic for deciding we're a UI client and therefore the newly sent message should be marked as read for its author, solving #440.

For some discussion on how to implement this, see #453.

[gnprice]

The value should probably say "ZulipMobile" somewhere

But this should be marked with a TODO(server-8) comment — once we're supporting only Zulip Server 8+, our use of read_by_sender (from #456) should always get respected, so we should no longer need to cater to that old heuristic and we can use a more transparent user-agent of our choice.