Add support for defining environment variables for all containers in a component

[amisevsk]

Description

Since devfile/api#149 will not be included in the Devfile 2.0 spec, we should consider supporting similar functionality via a controller-specific attribute on components. Some Che components (e.g. telemetry for the publicly hosted deployment) require the feature to function correctly, and reimplementing it as suggested in devfile/api#239 is difficult.

Options

1. Implement workspaceEnv as global variables

This would require

1. A plugin to define a devfile 2.0 global variable, e.g.

   template:
     variables:
       MY_ENV: "desired_value"

2. All plugins that expect to use the variable to have an environment variable defined

   env:
     - name: MY_ENV
       value: "{{MY_ENV}}"

However, the dependent plugins from step 2 cannot assume that the variable will be provided. If the plugin from step 1 is not included in a DevWorkspace, the DevWorkspace will issue a warning that there's no replacement for {{MY_ENV}}.

2. Implement workspaceEnv as an attribute

A plugin could define an attribute

attributes:
  workspaceEnv:
    name: MY_ENV
    value: "desired_value"

This would signal the controller to add the env var MY_ENV=desired_value to all containers in the DevWorkspace. However, this would be DevWorkspace controller-specific behaviour and completely outside the Devfile 2.0 spec as it's currently written.