feat(Validium): Abstract `GasAdjuster` struct into a trait

[ilitteri]

What ❔

Abstracts GasAdjuster struct into a trait which is implemented by new structs RollupGasAdjuster and ValidiumGasAdjuster

Why ❔

It was agreed in this comment https://github.com/matter-labs/zksync-era/pull/1017#discussion_r1481485699,

Checklist

• [x] PR title corresponds to the body of PR (we generate changelog entries from PRs).
• [x] Tests for the changes have been added / updated.
• [ ] Documentation comments have been added / updated.
• [x] Code has been formatted via zk fmt and zk lint.
• [x] Spellcheck has been run via zk spellcheck.
• [x] Linkcheck has been run via zk linkcheck.