Add OS-SIMPLE-CERT extension

This provides a very basic mechanism for retrieving the CA and signing
certificate from the identity service. It aims to simply replicate the
functionality provided by the v2.0 API.

SecurityImpact
Change-Id: I24c77188ebb05ef57b378798584e3829d55827f2
Partial-Bug: #1259011
Merge "Add OS-SIMPLE-CERT extension"