Make task implementation for async configurable

[drtheuns]

Projects that don't use open-telemetry or with more complex process propagation requirements can now implement it themselves by configuring the Task module to use.

[benwilson512]

I think this is a fine idea but we definitely want this to be compile time.

[drtheuns]

The first time Absinthe compiles (e.g. on CI) it will give a warning about the configured module not existing (yet). I haven't tried but it might be possible to get around this with apply. I personally don't think it's an improvement, but I could try it out if you'd like.