Group Content Cache Context

In the newer versions we refer to the entity between a group and its content as the "group relationship", this patch still uses the old "group content" naming pattern. Setting to Needs Work because of that.

Having said that, do you have an example of how this could come in useful? I'm having a hard time thinking of one. The only reason we have route.group is because of GroupRouteContext, which is used by the block that shows group operations.

Here's a version of this patch for Group 2.x

@Chizh273 I literally just gave some remarks on what needs to change in the MR yesterday and you reroll the patch for v2 when the MR for v3 isn't complete yet. I don't see how that adds value to the issue. While I appreciate people trying to help out with the issue queue, these type of patches make it harder to keep an issue clean and thus easy to review.

Faced with few cache issues on some blocks(group_content_menu) and custom breadcrumbs builder on project I was working a year ago(yeah it is a bit old issue). route.group was not enough, as for different group_content per same group cache wasn't purged. Migrating to newer module version wasn't possible.
As I said in the issue description, I'm not sure that this could be useful for other developers, anyways Group Content Cache Context could be done in custom module.