Explore, define and design the experience to use multiple values for app deployment in Kubeapps

[ppbaena]

Summary
The main goal of this issue is to set the foundation, identify the requirements, discuss the use cases and outline the technical design to use multiple/custom values for app deployment in Kubeapps.

Background and rationale
For a long time there have been several requests to save sets of values files (in a configmap, or CRD or something) and then be able to choose from them to apply during deployment, like templates (t-shirt sizes) you can save and reuse them. It is something that could be really useful but it is necessary to explore the feasibility and use cases to design it properly providing a standard experience across all packaging formats.

Description
Explore and define the scope for custom values feature in Kubeapps

Acceptance criteria

• Design document is available (problem statement, requirements, use cases, and a proposal for technical design).

[absoludity]

Updated the description, linking to the document which allows comments etc.