Use PEP 563 annotations

[adriangb]

I've noticed that the typing in the otel packages needs some love. It'd be nice to at least get the API and SDK touched up since those are used directly by users.

One thing I'd like to propose right off the bat is that we adopt PEP 563 (from __future__ import annotations) everywhere because it:

1. Helps with performance since type annotations aren't evaluated at runtime
2. Allows using more succinct syntax like dict[str, str] | None so less imports and boilerplate

This is a change that can pretty much be applied programmatically, things like pyupgrade and ruff can do it.

[ocelotl]

Sounds good, @adriangb, can you open a PR?

@aabmass this may be of your interest.