Refactor in-memory signing CAs to use a single implementation

[haydentherapper]

Summary

There is no difference between the CreateCertificate implementation for x509ca and intermediateca, so I did a few refactors to merge the two implementations:

• Moved intermediateca to baseca package
• Moved extensions.go and common.go out of x509ca, since x509ca is just for pkcs11, and this caused an import cycle
• Refactored x509ca and ephemeralca to embed baseca.BaseCA, removing the duplicate CreateCertificate/Root implementations
• Renamed x509ca to pkcs11ca
• Moved VerifyCertChain to common.go, since it didn't need to be a function of baseca

The primary benefit of this refactor is that all CA implementations that use the base CA implementation will support embedded SCTs. googleca is a special case.

Ticket Link

Fixes

Release Note

Refactored ephemeralca and pkcs11ca to support embedded SCTs

[codecov-commenter]

Codecov Report

Merging #644 (9b5ee7d) into main (ce8d2fb) will increase coverage by 0.87%.
The diff coverage is 85.05%.

@@            Coverage Diff             @@
##             main     #644      +/-   ##
==========================================
+ Coverage   62.02%   62.90%   +0.87%     
==========================================
  Files          29       28       -1     
  Lines        1646     1596      -50     
==========================================
- Hits         1021     1004      -17     
+ Misses        552      520      -32     
+ Partials       73       72       -1     

Impacted Files	Coverage Δ
pkg/ca/fileca/fileca.go	48.48% <ø> (ø)
pkg/ca/googleca/v1/googleca.go	52.25% <0.00%> (ø)
pkg/ca/baseca/baseca.go	56.09% <66.66%> (ø)
pkg/ca/common.go	85.93% <85.93%> (ø)
pkg/ca/ephemeralca/ephemeral.go	70.73% <100.00%> (+0.73%)	⬆️
pkg/ca/extensions.go	100.00% <100.00%> (ø)
pkg/ca/fileca/load.go	58.62% <100.00%> (-10.35%)	⬇️
pkg/ca/kmsca/kmsca.go	53.57% <100.00%> (ø)
pkg/identity/email/principal.go	90.90% <100.00%> (ø)
pkg/identity/github/principal.go	90.47% <100.00%> (ø)
... and 7 more

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[haydentherapper]

@dlorenc @bobcallaway Bumping for review. Each of the commits is a refactor.

[bobcallaway]

lgtm, sorry for the delay