[feature] Make rejection of empty user agents configurable

[bradanfleming]

Is your feature request related to a problem ?

It would be nice if GoToSocial could be configured to accept requests with empty User-Agent values. This doesn't have much use, but it would make it easier to rule it out as a cause of compatibility issues with other software. For example, BlueSky's AppView used an empty User-Agent until recently. bluesky-social/atproto#3504

Describe the solution you'd like.

I'd like some way to turn it off at runtime. I ended up using a config option (see bradanfleming@b2834c1), and I can make a PR for it if that works, but there might be a better way to go about it.

Describe alternatives you've considered.

I considered disabling it when built with DEBUG=1, but that seemed silly.

Additional context.

No response

[tsmethurst]

We might wanna at some point add this to some kind of "middleware stack config" thingy but I think making it an option on its own would be a bit silly. It's pretty reasonable to expect remotes to identify themself with at least a user agent imo. I'll whack a future tech label on this and we can consider it down the road if we ever make middleware more customizable in general!