[VIndex] Add binary to verify another Verifiable Index

[mhutchinson]

The request is for a command that:

• Takes:
  • URL of an Output Log
  • URL of an Input Log
  • MapFn
• Confirms:
  • Every entry in the Output Log can be verifiably reproduced, by building the equivalent index locally

Out of scope, but not prohibited, are:

• Publishing an output log for this copy
• Making cosignatures for any of the entries in the original Output Log available

The only output should be process failure with a specified error code and appropriate console logging if any entry in the Output Log could not be reproduced.

Later work may change this to not fail, and communicate errors via exported metrics.