At the moment, `HTTPClient`'s entire API surface violates Structured
Concurrency. Both the creation & shutdown of a HTTP client as well as
making requests (#807) doesn't follow Structured Concurrency. Some of
the problems are:
1. Upon return of methods, resources are still in active use in other
threads/tasks
2. Cancellation doesn't always work
This PR is baby steps towards a Structured Concurrency API, starting
with start/shutdown of the HTTP client.
Co-authored-by: Johannes Weiss <johannes@jweiss.io>
Cory Benfield
Johannes Weiss