feat: Validium mode

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What ❔

This PR adds the necessary changes for supporting Validium mode on the zksync-era side. For the moment, these are:

• Skipping some pubdata checks in Executor.sol when Validium mode is on.
• The PubdataPricingMode should be initialized correctly depending on the mode.

Why ❔

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.

TODOs

• Using PubdataPricingMode we can get rid of the use of the pre-processor.
• Passing the --validium-mode flag from zksync-era we can get rid of VALIDIUM_MODE env finally (the above point is required as we won't be using the env for the preprocessor).

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Superseded by #236