Allow custom integrations to behave like built-in Integration 2.0

Who would benefit?

Companies that use dev tools that aren't supported by Aha!

What impact would it make?

Removes lots of manual work and improves UX for custom integrations.

How should it work?

Allow adding custom integration with a name and icon.

Allow integration fields for custom integrations. Support URL field with text and link behind it (like Jira issue link).

When "Send to" is engaged, allow the webhook to provide a response that sets integration fields.

As a result, features and requirements will show the issue link in Integrations field the same way as it works with built-in integration. Should work with feature cards too.