Add FIPS section to USAGE-GUIDE.md

[lundinc2]

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Problem:

The USAGE-GUIDE.md does not have a section covering FIPS. We should add a section that explains how s2n-tls can be used for FIPS applications.

Solution:

Update USAGE-GUIDE.md with a FIPS section.

• Does this change what S2N sends over the wire? If yes, explain.
• Does this change any public APIs? If yes, explain.
• Which versions of TLS will this impact?

Requirements / Acceptance Criteria:

What must a solution address in order to solve the problem? How do we know the solution is complete?

• RFC links: Links to relevant RFC(s)
• Related Issues: Link any relevant issues
• Will the Usage Guide or other documentation need to be updated?
• Testing: How will this change be tested? Call out new integration tests, functional tests, or particularly interesting/important unit tests.
  • Will this change trigger SAW changes? Changes to the state machine, the s2n_handshake_io code that controls state transitions, the DRBG, or the corking/uncorking logic could trigger SAW failures.
  • Should this change be fuzz tested? Will it handle untrusted input? Create a separate issue to track the fuzzing work.

Out of scope:

Is there anything the solution will intentionally NOT address?