Add arbitrary key-value map data interface to pass data between plugins and API clients

Add arbitrary key-value map data interface to pass data between plugins and API clients

cert-svc allows to use validator plugins that can have any arbitrary logic embedded.
Installers may need to get special information, characteristic for plugins, from cert-svc.
New SignatureData class methods allow to set (on plugin side) and retrieve (on installer side)
simple string values that can have their meaning defined by plugin logic.

Installer should depend on specific plugin's key name definition (alternative is hard-coding)
and check for values using SignatureData interface.

Change-Id: I098a33b981d2b175ea72ba16cd8c968048ff116d