There are currently two types of BCCSP
providers:
- SW
- PKCS11

This CR addes a new "PLUGIN" type support
BCCSP providers implemented as Go plugins.
This is useful for providers who might
not be able to use PKCS11 as well as for
providers who want to use different
cryptographic algorithms that are not
supported via PKCS11 or the SW BCCSP.

Change-Id: I908634cbd4e6215ad682c97f5acc7a064bca643f
Signed-off-by: Gari Singh <gari.r.singh@gmail.com>