Removed indirect dependencies from go.mod

No major changes to the functionality. Removed indirect dependencies from go.mod.

support struct in referenced proto meaages

This release adds support for protobuf's with google.protobuf.Struct from referenced/imported proto files.
Following is an example.

message Action {
    string name = 1;
    google.protobuf.Struct parameters = 2;
}

message Execution {
    string name = 1;
    Action action = 2;
}

exclude fields from being indexed

This release adds support for excluding the fields from being indexed by google cloud datastore using a custom extension
in proto files.

Initial release.

This release mainly contains two functions

1. ProtoMessageToDatastoreEntity
   Translates a given proto message to datastore entity.
2. DatastoreEntityToProtoMessage
   Translates a given datastore entity to a proto message.