Implement plugin flattening for DevWorkspaces

[amisevsk]

Description:

• Implement basic plugins support (Plugin flattening #240)
• Support defaulting Web Terminal workspaces (add default component) (Enable referencing plugin components by ID (internal-registry only) #263)
• Support plugins specified by id

[sleshchenko]

The support for parents is moved to a dedicated issue.
I don't think it's important for Devfile 2.0 + Che or WTO.

[sleshchenko]

To avoid this issue be a long-long story, plugins with URI is moved to a dedicated issue as well #266