TextMap typeclasses for Java implementations?

[rossabaker]

Following from #207, would it make sense to have adapters from the Java implementations?

Why:

• Because we can
• There's a wealth of them in the java instrumentation library

Why not:

• Because we can doesn't imply we should
• It's probably usually the Java agent invoking those from the Java SDK for Java types in practice? Wait until someone wants it?

If we do:

• It should be explicit. ~~We don't know what's mutable and what's not from parameter A~~. [Edit: it's supposed to be mutable in that world, but it might not be canonical. Still better to be explicit.]

[iRevive]

Currently, we can convert Java's TextMapGetter and TextMapPropagator to otel4s. And wise-versa:

https://github.com/typelevel/otel4s/blob/main/oteljava/common/src/main/scala/org/typelevel/otel4s/oteljava/context/propagation/PropagatorConverters.scala

[iRevive]

It's not a typeclass, but the functionality is somewhat similar.

[rossabaker]

This seems good to me.