declarative query api to support property queries

**Proposal**
We would like to use an Artifact Type for Datasets, and another one for Datafiles, then query all DataFiles associated with a particular Dataset. However right now there is no way for me to perform this query.

We need the ability to query artifacts based on the property values, specifically I'm looking for ```get_artifacts_by_property_value``` where I can pass they key and value I'm looking for.

The reasoning behind this request is that Datasets can be composed of files, and in restricted projects we might be asked to query and explain the use of each individual file depending on its contents.

```
class DataFileType(object):
data_type = metadata_store_pb2.ArtifactType()
data_type.name = "DataFile_V1.0"
data_type.properties["data_file_name"] = metadata_store_pb2.STRING
data_type.properties["data_file_size"] = metadata_store_pb2.STRING
data_type.properties["data_file_created_by"] = metadata_store_pb2.STRING
data_type.properties["data_file_created_date"] = metadata_store_pb2.STRING
data_type.properties["data_file_parent_dataset"] = metadata_store_pb2.STRING
```

> Note: Another approach would be to allow for artifact hierarchies, though the approach above would solve the problem faster.