perf: add more parallelisation to gkr prover

[yoaveshel]

What ❔

Reduced proving time from ~54s to ~51s.

• Adds distribute_powers_parallel to fft
• Uses it in stage1 to parallelize coset LDE computation
• Precomputes combined_challenges = r1 * r2 in BaseFieldPolySourceAfterTwoFoldings to eliminate a redundant extension field multiplication on every get_at_index call during the two-folding step

Why ❔

Is this a breaking change?

• Yes
• No

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted.