feat: improve hyperkzg api

[JayWhite2357]

Rationale

There are a few things in hyperkzg that make upstream usage impossible or difficult. In particular, the actual proof is inaccessible in a transparent fashion.

Changes

Take a look at the individual commits.

• feat: make hyperkzg EvaluationArgument fields public
  • This allows upstream usage to access the actual proof directly. In particular, this is useful for custom seralization.
• feat: derive Default for hyperkzg ProverKey
  • This is convienient because the ProverKey is a placeholder for hyperzkg.
• feat: make hyperkzg prover pad polynomial with 0s
  • If hat_P.len() is not 1 << point.len(), the prove method currently panics. This commit pads hat_P with 0s instead, allowing for more flexible upstream usage.
• fix: handle empty point with hyperkzg
  • Currently, hyperkzg does not work if the point is empty. This commit simply defaults to using a trivial point. This default respects the commitment and evaluation.