feat: RequeueAtSignal #587

For many workflows which are time sensitive, requesting functionality to specify an absolute RequeueAt time instead of only relative delays.

Motivation

1. Time-sensitive workflows – Many workflows need to execute at specific absolute times (e.g., "run at 3:00 PM EST", "execute at market open", "process at end of business day").
2. Scheduled operations – Relative delays (ReQueueAfterSignal) are insufficient when the exact execution time matters, especially across time zones or when workflows are created hours/days in advance.
3. Precision scheduling – Absolute timestamps eliminate drift from relative delay calculations and ensure consistent execution times.

Current State

What Exists

Current Limitations

• ReQueueAfterSignal only accepts timedelta (relative delay)
• Must calculate relative delay from current time to target time
• Prone to timing errors if calculation happens at wrong moment
• No direct way to specify "requeue at 2025-01-15 14:30:00 UTC"

Proposed Solution

New Signal: RequeueAtSignal

Add a new signal class that accepts an absolute timestamp (datetime) instead of a relative delay.

from exospherehost import RequeueAtSignal
from datetime import datetime, timezone

# Requeue at a specific absolute time
target_time = datetime(2025, 1, 15, 14, 30, 0, tzinfo=timezone.utc)
raise RequeueAtSignal(target_time)

Key Features

1. Absolute timestamp support – Accept datetime objects with timezone awareness
2. Backward compatibility – Keep ReQueueAfterSignal unchanged
3. Validation – Ensure timestamp is in the future
4. Timezone handling – Support timezone-aware datetimes, convert to UTC internally