Add "self" link to point to the path of devfile in index schema

[GeekArthur]

Is your feature request related to a problem? Please describe.
According to this thread, for index server REST APIs, http://{registry host}/devfiles/{stack} is a convention we define in our spec, the proposal is we can add "self" field to point to the path of the devfile in index schema so that consumer can use http://{registry host}/{self} (where {self} is devfiles/{stack}) to get the content of the devfile at the time of getting the index.

Describe the solution you'd like
To make the change, I think the URL of the REST API design is still http://{registry host}/devfiles/{stack}, but if we do need a "self" as the shortcut for users we can the following things:

• Change the current "self" field to "image" or "stack"
• Add the "self" field to point to devfiles/{stack}
• Update index schema and index generator
• Update index server REST APIs

Which means the main URL definition is http://{registry host}/devfiles/{stack}, users can use either use http://{registry host}/devfiles/{stack} or http://{registry host}/{self} to get the content of the devfile, so that we can both provide a clear URL definition to users with RESTful design and let users get the devfile content with self placeholder without worrying about the structure.

CC @amisevsk

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