[#1477] Allow Orchard-only UFVK be created from an Orchard FVK

[pacu]

closes #1477
closes #1476

[conradoplg]

LGTM, though if someone tries to implement FROST with Sapling they will need a from_sapling_fvk so it might make sense to just expose new(), but let's wait for ECC input

[nuttycom]

utACK, this is an unstable development API addition. Ordinarily we might use a compilation flag for this, but the change is trivial enough that I think it's fine.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.82%. Comparing base (7f7b685) to head (22dd30f).
Report is 38 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1478   +/-   ##
=======================================
  Coverage   60.82%   60.82%           
=======================================
  Files         140      140           
  Lines       16472    16472           
=======================================
  Hits        10019    10019           
  Misses       6453     6453           

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