Sampling Propagation:Tag serializer

[marcotc]

This PR implements part of the [RFC] Trace Tags Propagation in Distributed Traces.

The work will be merged into the feat-Sampling-Propagation branch, like this PR.

This PR serializes and deserializes x-datadog-tags, when sending or receiving them through HTTP headers. The format is available in the RFC.

These tags will be received from upstream services through distributed trace propagation, and be send downstream through propagation injection.

Follow up PRs will actually use this serialize/deserializer.