Implement Certificate Transparency policy

[djc]

An attempt to make configuration of the certificate transparency policy more robust. This doesn't have testing yet; my first question here is if this should live in rustls itself or if it should be part of the sct crate.

(Feel free to take this and run with it/build on it/change it in whatever way seems best.)

Fixes #803.

[codecov-commenter]

Codecov Report

Merging #819 (e145ad6) into main (14c024d) will decrease coverage by 0.14%.
The diff coverage is 38.00%.

❗ Current head e145ad6 differs from pull request most recent head 5a64a2f. Consider uploading reports for the commit 5a64a2f to get more accurate results

@@            Coverage Diff             @@
##             main     #819      +/-   ##
==========================================
- Coverage   96.52%   96.38%   -0.15%     
==========================================
  Files          59       59              
  Lines        9307     9330      +23     
==========================================
+ Hits         8984     8993       +9     
- Misses        323      337      +14     

Impacted Files	Coverage Δ
rustls/src/builder.rs	91.48% <ø> (ø)
rustls/src/lib.rs	100.00% <ø> (ø)
rustls/src/verify.rs	70.37% <9.67%> (-4.28%)	⬇️
rustls/src/client/builder.rs	94.33% <83.33%> (-5.67%)	⬇️
rustls/src/verifybench.rs	100.00% <100.00%> (ø)

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[ctz]

Thanks, this is a good improvement.

With the benefit of hindsight, it was probably premature to implement anything CT related in rustls, because there seems to have been little deployment of the CT features in TLS elsewhere; instead everything is using them embedded in certificates.

[djc]

If so, should we make CT off by default (no need to call without_certificate_transparency_logs()), enabled by calling the builder method explicitly in one of the later stages?

[ctz]

If so, should we make CT off by default (no need to call without_certificate_transparency_logs()), enabled by calling the builder method explicitly in one of the later stages?

I think it would be an improvement if it were to default to off. We could revisit that once we have a complete CT implementation.