secrets/host: implement Create method for host plugin

[VinozzZ]

Implement a Create method so it matches with the new secret plugin interface in porter

[carolynvs]

Since the create method is just for Porter, and the rest of cnab-go wouldn't take advantage of it, let's see if we can make this change in just porter alone, and not edit cnab-go.

In Porter, we can create a new type that is composed of SecretStore and add the Create method on the wrapping type. Then Porter can use that wrapped type instead of SecreStore directly. Can you give that a try?

[VinozzZ]

Thanks for the suggestion. I will close this PR since this is only useful for porter