buffet: GCD command defintion. Compound object type support.

Added support for "object" type. Refactored parameter validation
to make sure we have object schema context when we validate
a value of parameter.

Parameter |schema| was used in two different contexts, as both
a base parameter definition and as a custom object definition.
Renamed the former to be 'base_schema' and latter as
'object_schema' to remove the confusion.

Extracted common data type manipulation functions into
schema_utils.cc/.h files.

BUG=chromium:374860
TEST=All unit tests pass.

Change-Id: I6c3549849a258bcc94b3d754acd14e072438d140
Reviewed-on: https://chromium-review.googlesource.com/204793
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>

buffet/commands/schema_constants.h

diff --git a/buffet/commands/schema_constants.h b/buffet/commands/schema_constants.h
index 0e58cff..087f53b 100644
--- a/buffet/commands/schema_constants.h
+++ b/buffet/commands/schema_constants.h
@@ -18,8 +18,11 @@
 extern const char kPropTypeChanged[];
 extern const char kUnknownType[];
 extern const char kInvalidPropDef[];
+extern const char kInvalidPropValue[];
 extern const char kNoTypeInfo[];
 extern const char kPropertyMissing[];
+extern const char kUnknownProperty[];
+extern const char kInvalidObjectSchema[];
 }  // namespace errors
 
 namespace attributes {
@@ -34,6 +37,10 @@
 extern const char kString_MaxLength[];
 
 extern const char kOneOf_Enum[];
+extern const char kOneOf_Metadata[];
+extern const char kOneOf_MetaSchema[];
+
+extern const char kObject_Properties[];
 }  // namespace attributes
 
 }  // namespace commands