[FEATURE] custom forms in cms-base

[mkucmus]

Description

Add ability to display and manage custom forms coming as a another part of CMS element.

If it's possible - add missing OpenAPI Schema in the extension's codebase to enable fetching it via api-client-next.

@kstala may I ask you for the details what acceptance criteria in this matter should look like?

Use Case

Currently we handle only contact form and newsletter types. Custom for is another one we should cover in cms-base package.

Proposed Solution

Follow our latest practice how to deal with external extensions if it's not a part of the core. Maybe consider providing an example instead of extending cms-base itself.

Alternatives Considered

No response

Additional Context

No response

[BrocksiNet]

Created a custom form for testing here in the default storefront.