feat: Add metronome source implementation

[ferenc-csaky]

Key Changes

• Added a new metronome connector that emits a monotonically increasing BIGINT sequence per second based on a wall-clock timestamp column
• Parallelism is forced to 1 so the metronome acts as a single global clock source
• Emits catch-up records when delayed by comparing current epoch seconds against the persisted start epoch second and last emitted sequence number
• Uses intended event timestamps for catch-up records, so delayed emissions still map to the correct event-time seconds
• State is carried through MetronomeSplit, preserving lastEmittedNumber and startTimestampSec
• MetronomeEnumeratorState also tracks any pending split, so returned splits are reassigned without losing progress
• Supports limit pushdown by creating a bounded runtime source when LIMIT is pushed into the table source
• Added schema validation requiring exactly two physical columns: BIGINT and TIMESTAMP/TIMESTAMP_LTZ

[mateczagany]

Good for now, but we should add tests for this later