util: add task builder API

[AlphaKeks]

Motivation

This PR implements a builder API for tokio_util::task::JoinMap as discussed in #7179.

Solution

JoinMap now has a public build_task method that returns a JoinMapBuilder. JoinMapBuilder lets you configure a name for the task and then spawn it on the JoinMap you originally got the builder from.

Because JoinMap contains a JoinSet and join_set::Builder borrows its JoinSet mutably, join_map::Builder cannot contain both a &mut JoinMap (which is required for inserting the task) and a join_set::Builder (which would borrow JoinMap::tasks). I see 2 solutions to this:

1. refactor JoinMap::insert in such a way that it is a free function instead of a method, and then make Builder contain the individual parts of JoinMap that are required
2. make Builder store all the same state that join_set::Builder (or task::Builder for that matter) stores

I chose to go with option 2, although in the future it may be better to switch to option 1, if task::Builder ever extends its API with callbacks and such.

The tokio_util::task::join_map module is currently also not public, so instead of mirroring join_map::Builder (where the join_map module is public), the Builder struct is re-exported as JoinMapBuilder. I'm not convinced this is the way to go, but making the module public would also require documenting it (which I am willing to do if desired).