Limiting scopes for certain grant_types

[rishabhsairawat]

Feature: To limit certain scopes to certain grant type.

Why?: We may not want all the scopes available for all the grant types. We may want certain scopes to be available only for certain grant types. For example, read-only scopes over the implicit grant.

@nbulaj What do you think about this?

[nbulaj]

Hi @rishabhsairawat . I think it can be useful, but requires some changes that must not break compatibility with previous versions of Doorkeeper. Also I think it must be good-configurable. Would you like to try to produce a PR?

[rishabhsairawat]

@nbulaj Sure.