[Dev UI] Span name for `ai.generate()` step

[lacolaco]

Overview

In building a multi-step flow, each ai.generate() step cannot be identified at the trace view because they all are labeled with generate.

I'd like to distinguish them with explicit naming, as well as ai.run().

User goal(s)

• A developer can set a name to ai.generate() step.

Requirements

Acceptance Criteria

• 1
• 2
• 3

Designs

• First draft: ai.generate({ prompt: '...', stepName: 'foobar' })

Notes

[marcelfolaron]

@pavelgj I think this can be closed now.