feat(s2n-quic): expose mtu provider

[toidiu]

Description of changes:

While it is currently possible to specify MTU setting on the IO provider (endpoint specific configuration), an application may want to specify these values per connection.

This PR exposes an MTU provider that allows Applications to provide MTU values at connection granularity. The MTU provider exposes remote_address and endpoint_config, which Applications can use to identify the connection.

Usecases:

By default s2n-quic will attempt to set the DF (dont fragment) and do MTU probing on a path. The only way to disable MTU probing is to set the initial, base, and max MTU to the same values. With the inclusion of the MTU provider, Applications can disable MTU probing at the connection level rather than the entire endpoint.

MTU probing can lead to higher path MTUs and therefore lead to better transmission efficiency (25% = (1500-1200)/1200). However, probing for higher MTUs takes time and therefore doesn't benefit Initial and Handshake packets. Applications operating on predictable networks could set the MTU values for known paths and also benefit from efficiency gains during connection establishment.

Testing:

• Unit tests to confirm that the MTU provider values propagate to the path.
• Unit test to confirm that validation is enforced.

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