Allow inserting into JoinMap directly

[AlphaKeks]

Is your feature request related to a problem? Please describe.
I'm using tokio's unstable API, mainly for tracing, and have started using tokio::task::Builder to name my long-running tasks. I now also have a use case for tokio_util::task::JoinMap, and find it unfortunate that the two do not compose. For my particular case, I'd just like to give names to the tasks in my JoinMap, but this can be generalized to other use cases as well, such as spawning a task manually in one place, and then inserting it into a JoinMap later (or conditionally).

Describe the solution you'd like
What I think would be a reasonable addition to JoinMap's public API is an explicit insert() method that takes a key and a JoinHandle.

Describe alternatives you've considered

• leaving the API as-is, because this may not be an important feature to many people
• adding more spawn_* methods that take a task::Builder or return their own builder type
  • this does not seem like a very good solution, as it adds way more code and is less general than what I propose, but it would fix my specific issue just as well

Additional context
JoinMap already has a private insert method that is called by spawn and friends, so the change may be as simple as making that method pub (and potentially changing the parameter type from AbortHandle to JoinHandle? unclear if that's necessary or desired).

[Darksonn]

Since JoinMap is implemented using JoinSet, this is a duplicate of #5924.

[AlphaKeks]

Since JoinSet does have a build_task method (one of the alternatives I listed), should JoinMap also expose an API like that?

[Darksonn]

That would be fine.