Add portable-atomic support to tracing-core

[bushrat011899]

Motivation

• Fixes portable-atomic support for alloc::sync in tracing-core #3173

Solution

• Added portable-atomic-util as an optional dependency gated behind a new feature, portable-atomic to tracing-core
• When portable-atomic is enabled, switched uses of Arc and Weak away from alloc::sync to portable_atomic_util.
• Added workaround for a lack of support for unsized coercion in custom types. I've included a comment linking to this issue explaining the missing functionality.

Notes

I'm currently working on no_std support for the Bevy Game Engine, which includes targeting embedded devices without full native support for atomics, such as the RP2040 Raspberry Pi Pico. Using portable-atomic, users can provide a fallback implementation for atomics to libraries (typically through critical-section), allowing libraries to depend on atomics even on unsupported platforms. This PR would allow using tracing on more platforms without sacrificing ergonomics or performance on existing ones.

This is my first time contributing to tracing, so please reach out if there's anything I can do to help with reviewing and merging this change!

[BD103]

Changes look good to me!