Add privilege that user can get only role assigned to him.

[yoh1496]

Related to personium/app-cc-home#263 .

social-read make user can read all roles about target cell.

[shimono]

This is important because social-read is too strong privilege to give to visitors in general.
First we need to design external spec.

[shimono]

Let us read https://datatracker.ietf.org/doc/html/rfc3744.html
and see if there are any suitable api to extend to achieve this goal.

[shimono]

This API looks good?
https://datatracker.ietf.org/doc/html/rfc3744.html#page-22