feat(s2n-quic): expose configuration options for congestion control

[toidiu]

Description of changes:

This PR exposes a few configuration options that modify the default congestion controllers.

While it is possible to create a custom congestion controller, customers might find it more useful to simply override some key parameters on the deafult congestion controller algorithm rather than replicating the entire congestion control algorithm.

Call out:

• The with_congestion_window API is exposed as a stable API, while other options are marked as unstable
  • While it makes sense to expose CC configuration parameters behind an unstable flag (getting them wrong can render the network impaired), setting the initial congestion window (initcwnd) is also supported on TCP and seems like a common enough use case.

Testing:

The new options should not affect the default values and therefore existing tests should continue to pass.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[WesleyRosenblum]

thanks for putting the default values!