Support SEC1-format EC keys via PKCS8 conversion

[ctz]

This has been asked for 5-6 times over the past couple of years (including internally at my day job), so I think it's overdue for being supported. See, eg, #997, #332, #767, #245, #409 -- plus a number of downstream bugs.

While briansmith/ring#1456 (or equivalent) awaits merging, we'll achieve this in the meantime by chucking the PKCS8 outer layer around the SEC1 encoding.

[codecov-commenter]

Codecov Report

Merging #998 (fe64924) into main (f9a3d42) will increase coverage by 0.03%.
The diff coverage is 97.43%.

@@            Coverage Diff             @@
##             main     #998      +/-   ##
==========================================
+ Coverage   96.25%   96.29%   +0.03%     
==========================================
  Files          60       60              
  Lines        9748     9785      +37     
==========================================
+ Hits         9383     9422      +39     
+ Misses        365      363       -2     

Impacted Files	Coverage Δ
rustls/src/sign.rs	95.23% <97.36%> (+2.47%)	⬆️
rustls/src/x509.rs	100.00% <100.00%> (ø)

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