spec,libcni: add support for injecting runtimeConfig into plugin stdin data

spec,libcni: add support for injecting runtimeConfig into plugin stdin data

Add a new CapabilityArgs member to the RuntimeConf struct which runtimes can
use to pass arbitrary capability-based keys to the plugin.  Elements of this
member will be filtered against the plugin's advertised capabilities (from
its config JSON) and then added to a new "runtimeConfig" top-level map added
to the config JSON sent to the plugin on stdin.

Also "runtime_config"->"runtimeConfig" in CONVENTIONS.md to make
capitalization consistent with other CNI config keys like "cniVersion".