pass treeSize and rootHash to avoid trillian import

[bobcallaway]

Summary

Redefines util.CreateAndSignCheckpoint to take treeSize and rootHash as arguments to avoid adding dependency on github.com/google/trillian.

[codecov]

Codecov Report

Merging #1513 (aa96f2d) into main (55a5a33) will not change coverage.
The diff coverage is 87.50%.

@@           Coverage Diff           @@
##             main    #1513   +/-   ##
=======================================
  Coverage   66.94%   66.94%           
=======================================
  Files          83       83           
  Lines        8406     8406           
=======================================
  Hits         5627     5627           
  Misses       2103     2103           
  Partials      676      676           

Flag	Coverage Δ
e2etests	48.31% <87.50%> (-0.03%)	⬇️
unittests	47.78% <50.00%> (ø)

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Impacted Files	Coverage Δ
pkg/api/tlog.go	31.57% <50.00%> (ø)
pkg/api/entries.go	64.87% <100.00%> (ø)
pkg/util/checkpoint.go	77.77% <100.00%> (ø)
pkg/witness/publish_checkpoint.go	89.21% <100.00%> (ø)

... and 1 file with indirect coverage changes