Add `attributes` free-form map field for tool-specific fields in a number of places

[davidfestal]

Is your feature request related to a problem? Please describe.

How to support tool-specific fields in a number of places in Devfile 2.0 ?

Describe the solution you'd like

During the Devfile 2.0 early discussions, we had agreed that there should be an attribute free-form string map at a number of places in the top-level elements (commands, components, projects), to allow tools to provide tool-specific information.
This would be done the same as what already exists for Endpoints
This would be good to have that before the 2.0.0 release.

[davidfestal]

For now it was decided to add those attribute fields in the main objects (GO structs) that appear in lists:

• Command
• Component
• Project
• StarterProject ?

[elsony]

I'd say yes for StarterProject as well. I don't see any reason for excluding StarterProject.

[elsony]

Based on the discussion, we are moving this to 2.1 to discuss together with how we associate commands to projects in a single devfile.

[elsony]

Can this be closed now given that PR #214 has been merged?