Create a way to define a group of custom codecs that can be applied to the entire message

Add the ability to define a whole group of codecs that apply to the message fields. For example:

Old:
message SomeDcclMessage
{
    optional int32 a = 1 [ (dccl.field).codec = "_v3"];
    optional int32 b = 2 [ (dccl.field).codec = "_v3"];
    optional double c = 3 [ (dccl.field).codec = "_v3"];
}

New:
message SomeDcclMessage
{
    option (dccl.msg).codec_group = "_v3";

    optional int32 a = 1;
    optional int32 b = 2;
    optional double c = 3;
}

The codec group applies a set of codecs in place of the defaults to the entire message. In one case, we can use it to improve the default codecs while maintaining backwards compatibility with Goby DCCLv2. Also, users could apply a bunch of custom codecs to a given message while eliminating the extra text involved with adding 'codec = "some_custom_codec" to each field in the message.