Consistently use cert pointer in verification

[masih]

The finality certificate APIs at times switch between pointer and values. This introduces the need for extra gymnastics when validating certs, where a slice of cert pointers need to be converted to values to use the cert verification APIs.

Instead, consistently accept pointers to certs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.40%. Comparing base (c92e6d4) to head (a64e637).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #751      +/-   ##
==========================================
- Coverage   69.80%   69.40%   -0.41%     
==========================================
  Files          74       74              
  Lines        7497     7497              
==========================================
- Hits         5233     5203      -30     
- Misses       1856     1880      +24     
- Partials      408      414       +6     

Files with missing lines	Coverage Δ
certexchange/polling/poller.go	79.38% <100.00%> (ø)
certs/certs.go	92.68% <100.00%> (ø)
host.go	62.66% <100.00%> (-3.90%)	⬇️

... and 4 files with indirect coverage changes

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