Make re-usable one-of value structure in protobuf to ease programming

[jbrzozoski]

In the current protobuf definition, there are four separate oneof value blocks which are all nearly identical. Depending on the application language being used, this can required nearly identical repeated code blocks to handle reading and writing them. I think there is a strong argument that we create a common block something like this and reuse it throughout the protobuf, with the understanding that certain data types are illegal to use within certain cases:

message TypedValue {
    optional enum DataType type = 1;
    optional bool is_null = 2;
    oneof value {
        uint32   int_value                      = 3;
        uint64   long_value                     = 4;
        float    float_value                    = 5;
        double   double_value                   = 6;
        bool     boolean_value                  = 7;
        string   string_value                   = 8;
        bytes    bytes_value                    = 9;       // Bytes, File
        DataSet  dataset_value                  = 10;
        Template template_value                 = 11;
        TypedValueExtension extension_value    = 12;
    }
}

The exact list of options in the oneof block should be expanded to be the superset of all possible oneof value blocks, obviously.

One other tweak in that sample, is that I used the explicit enum DataType instead of a simple integer. I suspect this will make handling of datatypes in the most languages source code more easy to understand as well.

[wes-johnson]

This is now done via this PR: #552