Support for scheduled relationships

[tenkoverse]

Problem Statement

SpiceDB currently supports expiring relationships ("give access from now until Y"), but not:

1. "Give access from X (future time)" (scheduled)
2. "Give access from X until Y" (scheduled and expiring)

This forces users to either:

• Use caveats (with the complexity/performance issues that led to native expiration support) by hoisting time management complexity into the application.
• Build external scheduling systems to write the relationships at a specific time

Similar design arguments that justified moving from the caveat-based expiration solution to native expiration times apply to this idea.

Solution Brainstorm

Extend the idea of optional expiring relationships to also support optional scheduled relationships:

• Add OptionalScheduledAt field alongside OptionalExpiresAt
• Relationships would be "invisible" until their scheduled time, then activate normally
• They can be used in addition to OptionalExpiresAt to create active time ranges.