add NewWithReadyData() method for stringlabels

[wanghaao]

If the third-party storage system has encoded the labels-data basing varint format, so we need a direct method to create the stringlabels's object or change the "data" field to be public.

[machine424]

Could you tell us more about how the encoding is done on that 3rd party? what it takes as input and why the utils from labels_stringlabels.go aren't/cannot be used

[bboreham]

This looks like something best kept in your own fork.
Note that we will hopefully move Prometheus to a different format, tag dedupelabels, after some issues are sorted out.