Expose lower level AF_XDP socket functions

[OliverGavin]

Problem:

Lower level AF_XDP socket functions are private and cannot be used directly (they are used within s2n).

Solution:

Make functions such as Channel::for_each public so they can be used without s2n specific layers above it.

The ask here is a short term solution until a wider effort is made to pull the lower level AF_XDP socket implementations into its own crate: #2403

• Does this change what s2n-quic sends over the wire?
  • no
• Does this change any public APIs? -->
  • not directly, but in migrating to it there will be changes

Requirements / Acceptance Criteria:

• Related Issues:
  • Split AF_XDP socket implementation from s2n-quic-xdp into its own project/crate #2403
• Will the Usage Guide or other documentation need to be updated?
  • No, not until AF_XDP sock is offered as a stand alone crate?
• Testing: How will this change be tested? Call out new integration tests, functional tests, or particularly interesting/important unit tests.
  • Scope is just making function public so additional testing should not be required