Implement cert chain generator/verifier from EC

[masih]

Implement testing utilities to:

• generate random finality certificate chain given an ec.Backend
• verify conformity of an existing chain with a given ec.Backend

Fixes #743

[Kubuxu]

Very rough review, but LGTM.

[Stebalien]

Nit: put this into an internal package?

[masih]

Nit: put this into an internal package?

I need to use it in lotus to verify the cert chain of a network, e.g. calibnet.
Because lotus ec.Backend implementation lives in Lotus.

[Stebalien]

Ah, I see. We're trying to unify our testing code and our actual code. Makes sense.

[codecov]

Codecov Report

Attention: Patch coverage is 64.10256% with 112 lines in your changes missing coverage. Please review.

Project coverage is 69.27%. Comparing base (b4e2522) to head (bc06f9a).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
certchain/certchain.go	62.81%	70 Missing and 33 partials ⚠️
certchain/options.go	74.28%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #775      +/-   ##
==========================================
- Coverage   69.82%   69.27%   -0.56%     
==========================================
  Files          74       76       +2     
  Lines        7557     7869     +312     
==========================================
+ Hits         5277     5451     +174     
- Misses       1869     1965      +96     
- Partials      411      453      +42     

Files with missing lines	Coverage Δ
certchain/options.go	74.28% <74.28%> (ø)
certchain/certchain.go	62.81% <62.81%> (ø)

... and 5 files with indirect coverage changes