Add c'tor to create ProofBuilder using only tree size

[AlCutter]

This PR adds a second c'tor for ProofBuilder which enables construction of a new instance based solely on the tree size.

This provides a means to construct consistency proofs without having to hold both checkpoints (e.g. if you're trying to convince someone else of consistency between a checkpoint they hold, and a newer checkpoint you hold - this is exactly what happens during the tlog-witness protocol).

This is safe as generated proofs must be verified against checkpoint root hashes by the consumer anyway; any inconsistency will be detected at that point.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 22.94%. Comparing base (07aa77d) to head (5ad1db7).
Report is 182 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (07aa77d) and HEAD (5ad1db7). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (07aa77d)	HEAD (5ad1db7)
	4	2

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #277       +/-   ##
===========================================
- Coverage   55.99%   22.94%   -33.06%     
===========================================
  Files           8       16        +8     
  Lines         759     1896     +1137     
===========================================
+ Hits          425      435       +10     
- Misses        265     1390     +1125     
- Partials       69       71        +2     

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