Allow TLS PSK without server certificate

[WillChilds-Klein]

Description of changes:

While we currently support TLS pre-shared keys (PSKs) for TLS 1.2, our
python integration tests have shown that we don't currently support TLS 1.2
PSKs on the server side unless a certificate has been configured. PSKs can be
used for both secret establishment and authentication, so there likely exist
legitimate use-cases for establishing PSK connections without certificates.
CPython's integration tests expect this behavior.

This gap appears to be incidental, not intentional. Unrelated OCSP work
introduced a requirement that a valid certificate public key is loaded on the
server before we sort out handshake parameters. If that load fails, the
hanshake is aborted before PSK can be negotiated for secret establishment. To
fix this, we simply check whether the server has a PSK callback enabled, and
allow the certificate loading to fail if so. We also handle the potential
nullity of the public key.

Call-outs:

This PR only applies to TLSv1.2 PSK. TLSv1.3 moved PSK negotiation earlier in
the handshake to client/server hello's, so does utilize this code path.
Currently, we only support TLSv1.3 PSK for session resumption, _not "pure" PSK
used for initial shared secret establishment. I have a branch where I'm
implementing this, but it's non-trivial.

We won't be able to delete the TLSv1.3 PSK python test patch until we implement
"pure" TLSv1.3 PSK.

Testing:

• CI

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.74%. Comparing base (39b3fae) to head (c0f062e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2083      +/-   ##
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- Coverage   78.75%   78.74%   -0.01%     
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  Files         598      598              
  Lines      103650   103652       +2     
  Branches    14718    14719       +1     
==========================================
- Hits        81633    81625       -8     
- Misses      21366    21372       +6     
- Partials      651      655       +4     

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