refactor(s2n-quic-core): simplify clock and timer traits

[camshaft]

Description of changes:

While implementing the AF_XDP IO provider, I wanted to make a generic event loop implementation so I didn't have to copy/paste the one from the tokio provider. Today, the clock to get the current time is generic. However, there's no trait for getting a timer from the clock and awaiting on its expiration.

In this change, the clock and timer traits are refactored to make it possible to use in the IO provider event loop. This is done by adding a ClockWithTimer trait that is used in the IO event loop.

Call-outs:

While I was changing the interface, I went through and removed all references to s2n-quic-platform in s2n-quic-transport with the testing module in s2n-quic-core. This simplifies the testing dependencies a bit and makes it so every crate can use the clock in a consistent way. The only last place that uses the time module is the default address token provider, and I've opened an issue to track that separately: #1738.

Testing:

Since this is just shuffling things around, the existing tests should cover the changes.

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