Add Custom Headers Support to "Start Workflow" Form in Web UI

[vswom]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I need to manually start a workflow from the Temporal Web UI that depends on custom headers for context propagation (e.g., x-tenant-id in multi-tenant applications).

Currently, the Web UI allows me to specify Workflow ID, Task Queue, Input, Memo, and Search Attributes, but there's no way to add custom headers. This means I cannot test or manually trigger workflows that rely on header-based context without writing separate scripts or using the CLI, which defeats the purpose of having a user-friendly Web UI.

Describe the solution you'd like

I would like to see a "Headers" section added to the "Start Workflow" form in the Web UI, similar to the existing "Memo" and "Search Attributes" sections.

The interface could accept a JSON object where keys are header names and values are header values (strings):

{
  "x-tenant-id": "tenant-123",
  "x-user-id": "user-456"
}

This would allow users to specify custom headers when starting workflows, just like they can do programmatically via the SDK:

await client.workflow.start(myWorkflow, {
  taskQueue: 'default',
  workflowId: 'my-workflow-id',
  args: [{ data: 'value' }],
  headers: {
    'x-tenant-id': 'tenant-123'
  }
});

Describe alternatives you've considered

1. Using Memo: This doesn't work because workflow interceptors and middleware typically read from headers via workflowInfo().headers, not from memo
2. Using Search Attributes: Same limitation as memo, plus search attributes have type constraints and are meant for querying, not context propagation
3. Using Temporal CLI: This works (temporal workflow start --header "x-tenant-id=tenant-123"), but requires CLI access and is less convenient than the Web UI
4. Creating wrapper API endpoints: This adds unnecessary infrastructure complexity just to work around a UI limitation

Additional context

Many modern applications use header-based context propagation for cross-cutting concerns like tenant identification, tracing, authentication, etc. This pattern is common in multi-tenant SaaS applications and is inspired by standards like OpenTelemetry's baggage propagation.

Supporting custom headers in the Web UI would:

• Align the Web UI capabilities with what's possible via the SDK
• Improve developer and operations team productivity
• Make the Web UI a complete tool for workflow management, not just workflow monitoring

This feature would be particularly valuable for testing, debugging, and operational scenarios where manual workflow triggering is needed.

[gow]

@vswom I've transferred the issue to our UI repo.