Support alternative async runtimes

Technically possible today with Tokio still used for retries, but we should be able to entirely remove Tokio as a required dependency.

Start by tracing where Tokio is required, including its use for retries, and identify the boundaries that prevent alternative async runtimes. Done means Tokio is no longer a required dependency while retries and the SDK continue to work with supported alternative runtimes.

Written by the indexing model from the issue text.