Allow configuring project-level provider settings within onboarding flow for self-managed provider

Background

This issue is split from Allow configuring project-level provider settin... (#451315 - closed), to allow implementing smaller chunks.

Problem to solve

When a customer tries to onboard product analytics and select a self-manged provider, if they do not have their project-level settings correctly configured we currently redirect them, to the settings page, where they need to enter settings then manually find their way back to the onboarding flow.

Proposed solution

Add a settings form directly within the onboarding flow to keep the customer in context.

Implementation plan

• add new Vue settings form for configuring project values
• add validation (required fields - all or none)
• wire up new form to modal within onboarding flow
• remove redirects to settings page
• update tests
• update docs