Implement message group configuration validation

• Validation is optional, but automatically performed if the metayaml library is available. Added it to suggest section in composer.json.
• All twn Yaml files are validated under 200 ms, which is fast enough in my opinion.
• Moved some code out of TranslateYaml to MessageGroupConfigurationParser where I also placed the new validation code.
• Added MetaYamlSchemaExtender interface, which I unfortunately can't use because there is no function to check whether a class implements and interface. There are functions that provide the information but they are harder to use and could be slower, so I went for duck typing instead. Left the class in, however, in case this wll change in the future as well as for other use cases. Constructing these objects is non-trivial, in case you wonder why I don't use instanceof.

Bug: T86000
Change-Id: I09e481d30d068177db3f71d2ebfffa87c394060c