chore(anvil): clean-up impersonated tx building

[mablr]

Motivation

Replace the ad-hoc pattern of constructing a bypass signature externally and passing it to the generic build_typed_transaction with a oneshot build_impersonated helper that owns the zeroed signature internally.

• Remove Ethapi::impersonated_signature
• Eliminate unnecessary Result wrapper
• Ensures all call sites use a consistent signature rather than constructing it independently.

... tl;dr less code 😄