add PipelineConfig to api to re-implement pipeline-level config

KFP v1 supported setting pipeline-level configuration via a
`PipelineConf` class. This class was deprecated and no replacement
was added to KFP v2.

add new PipelineConfig message in the api to support setting
pipeline-level configuration in KFP v2.

Signed-off-by: Greg Sheremeta <gshereme@redhat.com>

api/v2alpha1/pipeline_spec.proto

@@ -1091,11 +1091,17 @@ message SinglePlatformSpec {
   // Arbitrary configuration, which will be defined by the platform
   // protos/libraries.
   google.protobuf.Struct config = 3;
-}
 
+  PipelineConfig pipelineConfig = 4;
+}
 
 message PlatformDeploymentConfig {
   // Map of executor label to executor-level config
   // Mirrors PipelineSpec.deployment_spec.executors structure
   map<string, google.protobuf.Struct> executors = 1;
 }
+
+// Spec for pipeline-level config options. See PipelineConfig DSL class.
+message PipelineConfig {
+  // TODO add pipeline-level configs
+}