Benchmarking: Add pov_mode to V2 syntax (#3616)

Changes:
- Port the `pov_mode` attribute from the V1 syntax to V2
- Update `pallet-whitelist` and `frame-benchmarking-pallet-pov`

Follow up: also allow this attribute on top-level benchmark modules.

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: command-bot <>