Support Simple Forms again

[absoludity]

Summary
With the release of Kubeapps 2.6.0, we removed the simple forms support due to the removal of the form schema data that had to that point been included in the Bitnami catalog for some charts.

I'd like to be able to bring back support for simple forms by supporting custom form schema data in charts.

Background and rationale

As per the docs for the simple forms,

The goal of this feature is to present the user with the most common parameters which are typically modified before deploying a chart (like username and password) in a more user-friendly form.

Although the replacement table-based form data is useful, it is still a lot of information so misses the previous goal. Given that Kubeapps users often used this feature by adding their own form schema data to custom charts, the removal of this data from the Bitnami charts should not stop people from using this great feature.

With the new support for custom files in charts, we have a chance to enable a form-schema.json or similar to be recognised in a chart package and, when provided, used to display the simple form (IMO).

Acceptance criteria
If a chart includes a form-schema.json, this will be used by default to present the deployment options.

Additional context

We'd need to decide on a standard format for the form data, as well as the rendering library etc. (assuming we don't want to re-add the old custom items... to be decided).