feat: Add extra-platforms feature for non-atomic targets

[ericschaal]

Introduce a new Cargo feature, extra-platforms, to add support for hardware targets that lack native atomic CAS operations.

The library relies on AtomicU8, which is not available on all platforms (e.g., certain embedded targets like riscv32i or thumbv6m). This prevents the crate from being used in those environments.

This change "poly-fills" the required atomic types by adding an optional dependency on the portable-atomic crate.

I'm using a similar approach to the bytes crate.

[faern]

This sounds good to me! Sorry for the slow review. Would you be willing to rebase this to resolve the conflicts? Primarily the imports moved to a dedicated atomic.rs now