feat(controller-admin-jobs): add trigger support to job creation api

[shiyasmohd]

Enable explicit trigger configuration (OneShot, Interval, Cron) for all job types through the admin API, with OneShot as the default.

• Add trigger field to CreateJobRequest variants and JobDescriptor constructors
• Always serialize trigger in job descriptor, defaulting to OneShot when omitted
• Explicitly set Trigger::OneShot in dataset deploy handler
• Add trigger field to admin-client CreateJobRequest::Gc
• Add integration tests for all trigger types (OneShot, Interval, Cron)

[LNSD]

Before merging this, we need to handle the trigger unregistration flow. Otherwise, we'll be accumulating tokio tasks