[Brainstorm] Declare configurable environment variables in project's devfile

Description

This came up while discussing the relationship with the upcoming Secrets feature and Workspaces

Big idea: What if a .devfile could declare the environment variables it needs. When a user starts a workspace, we'll read the .devfile and show a form of configuration environment variables before the user starts the workspace:

• Users can enter hardcoded values for the needed environment variables found in the relevant .devfile.
• Users can point environment variables to stored secrets based on the relevant project.
• We can automatically map environment variable names to stored secret keys to improve usability. These are just defaults though. The user can change them. For example, the user can point the environment variable ACCOUNT_ID which was defaulted to the project's ACCOUNT_ID secret to a different secret named PAULS_ACCOUNT_ID.

This information will be included in the payload to actually start up the workspace.

/cc @ericschurter @oregand @cwoolley-gitlab @jmiocene