Feat: manual variables tab frontend part one

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What does this MR do and why?

• Part one for !158176 (closed) according to the comment !158176 (comment 1995151744)
• Introduces manual variables EmptyState, VariableTable and their Container.

Feat: manual variables tab frontend part one

• Add component for manual variables empty state.
• Add component for manual variables table.
• Add component for organizing those components above.

Changelog: added

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