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".

libcni/conf.go

@@ -201,22 +201,24 @@ func LoadConfList(dir, name string) (*NetworkConfigList, error) {
 	return ConfListFromConf(singleConf)
 }
 
-func InjectConf(original *NetworkConfig, key string, newValue interface{}) (*NetworkConfig, error) {
+func InjectConf(original *NetworkConfig, newValues map[string]interface{}) (*NetworkConfig, error) {
 	config := make(map[string]interface{})
 	err := json.Unmarshal(original.Bytes, &config)
 	if err != nil {
 		return nil, fmt.Errorf("unmarshal existing network bytes: %s", err)
 	}
 
-	if key == "" {
-		return nil, fmt.Errorf("key value can not be empty")
-	}
+	for key, value := range newValues {
+		if key == "" {
+			return nil, fmt.Errorf("keys cannot be empty")
+		}
 
-	if newValue == nil {
-		return nil, fmt.Errorf("newValue must be specified")
-	}
+		if value == nil {
+			return nil, fmt.Errorf("key '%s' value must not be nil", key)
+		}
 
-	config[key] = newValue
+		config[key] = value
+	}
 
 	newBytes, err := json.Marshal(config)
 	if err != nil {