LinkedIn: lead_generation objective with a lead form attached on create

What we're asking for: goal "lead_generation" for LinkedIn on POST /v1/ads/create, with a leadFormId to attach the form - the way leadGenFormId already works for Meta.

Why this matters to us: lead generation is the main reason our customers advertise on LinkedIn at all. B2B advertisers want a form in the feed, not a click to a website. Your lead-forms endpoints already create and list LinkedIn forms (we verified this works on our own account on 2026-08-28), and the leads can be read back - the only
missing link is connecting a form to a campaign.

What happens today: the supported goals for LinkedIn on create are engagement, traffic, awareness and video_views. There is no lead_generation objective and no way to attach an existing lead form, so a LinkedIn lead-gen campaign can only be built in Campaign Manager.

What "done" looks like for us: create a LinkedIn lead-gen campaign with an existing leadFormId in one call, see the form reflected on the ad when we read it back, and collect the submissions through the lead endpoints we already use.