Eager load post.user.groups relation and allow extensions to eager load relations

[SychO9]

Part of flarum/framework#2637
Works with flarum/framework#2724

Allows other extensions to eager load relations on its listing endpoint (like tags see flarum/tags#125).
Also eager loads post.user.groups when post.user is loaded, because the groups are called in policies for isAdmin checks.

[askvortsov1]

There's a lot less queries, but response time seems almost doubled. Is this just a unrepresentative sample, or is there something else going on? If these were being loaded one at a time, that feels like it should be more queries, right?

[tankerkiller125]

My guess is that the extra load time is coming from inefficient queries that the eloquent library generated if it is a good sample.

[SychO9]

That time is not a factor we should look at, it varies all the time depending on the local dev machine (and my machine struggles a bit), if I keep on visiting the endpoints the loading time will be faster, but if I hit the endpoint after a while, it'll be slower, most likely has to do with the apache server, so it's a different story on production environments.

For example, here's another test after hitting the end point consequently