feat: Schedule Support

[YunchuWang]

Schedule Support for Durable Task Framework
Overview
This PR introduces comprehensive scheduling capabilities to the Durable Task Framework, enabling users to create and manage recurring task executions with fine-grained control over timing and execution parameters.
Key Features

1. Schedule Management
   Create, pause, resume, update, and delete scheduled tasks
   Flexible schedule configuration with support for:
   Custom intervals (minimum 1 second)
   Start and end times
   Immediate or delayed first execution
   Custom orchestration instance IDs
   Orchestration input parameters
2. Schedule Configuration
   ScheduleCreationOptions for initial setup with required parameters:
   scheduleId: Unique identifier for the schedule
   orchestrationName: Target orchestration to execute
   interval: Time between executions
   Optional parameters:
   startAt: Custom start time
   endAt: Schedule termination time
   orchestrationInput: Input data for the orchestration
   orchestrationInstanceId: Custom instance ID
   startImmediatelyIfLate: Controls immediate execution behavior
3. Schedule State Management
   Robust state transitions with validation
   Schedule status tracking (Active, Paused, etc.)
   Execution history tracking:
   Last run time
   Next scheduled run time
   Execution token for operation validation
4. Client Interface
   Clean and intuitive client API through ScheduledTaskClient
   Async operations for all schedule management functions
   Comprehensive error handling and logging
   Support for cancellation tokens
5. Worker Integration
   Simple worker configuration through extension methods
   Automatic registration of required components:
   Schedule entity
   Schedule operation orchestrator

[YunchuWang]

fyi branch for e2e tests https://github.com/microsoft/durabletask-dotnet/tree/wangbill/schedule-e2e-tests. i added 32 testcases all passing

[sebastianburckhardt]

LGTM.

(I did not review the tests)

changes are updated based on feedback, thanks

[cgillum]

Added some comments. I'm particularly concerned about the possibility of null-ref exceptions in the Schedule.cs code. We're overriding the nullable reference types checks in ways that seem to open the door for lots of null-ref exceptions.