This is a continuation of #3906. Rich parameters will make the following possible:

• Users can edit parameters on existing workspaces
• Users are prompted when they update their template and new parameters are added
• Template authors can mark some parameters as immutable, meaning they cannot be changed after initially set. For example: changing the region of a workspace may destroy the volume!
• Icons, friendly names, and descriptions for parameters
• Pre-plan validation on parameters (e.g. validation errors on the input)

Eventually, rich parameters may allow us to add:

• Special validation/data sources (e.g. "all AWS regions," "all images in registry.mycompany.com")

Call notes:

• First, we'll introduce the new parameter data source
• Extend CLI to also use rich parameters
• Convert examples to the new format

Next:

• Add migration docs for migrating from variables
• Consider deprecating parameters

Vision:

• Terraform variables are template-wide only. In the "template settings" UI, you can edit "variables"