feat(slack/case): allow page command in dedicated case channels

[whitdog47]

This pull request introduces a new feature that allows the /dispatch-engage-oncall command to be used within dedicated case channels in Slack. This enhancement is designed to streamline the process of engaging on-call services directly from case channels, improving response times and coordination during case management.

Key Changes

• Case Engage Oncall Flow: Added a new function case_engage_oncall_flow in src/dispatch/case/flows.py to handle the engagement of on-call services for cases. This function includes logic to page the on-call service if specified.
• PagerDuty Plugin: Updated the page function in src/dispatch/plugins/dispatch_pagerduty/plugin.py and src/dispatch/plugins/dispatch_pagerduty/service.py to support the new event type "case" for paging.
• Slack Integration: Modified handle_engage_oncall_command and handle_engage_oncall_submission_event in src/dispatch/plugins/dispatch_slack/incident/interactive.py to support the new case flow. The command is now available for cases with dedicated channels.
• Code Refactoring: Adjusted existing logic to accommodate the new feature, ensuring seamless integration with current incident management workflows.