[BETA] Full client-side aggregate with histogram, distribution and timing

[hush-hush]

full client side aggregation for histogram and distribution. This feature use the new beta protocol 1.1 for dogstatsd allowing sending multiple values per message.

This also add the WithAgentMinVersion setting to trigger feature link to the agent version. For now only client side aggregation for histograms, distribution and timing which requires agent >=7.25 or >=6.25&&<7.0.0.

[ogaca-dd]

I did a quick review and it looks good to me 👍

WithAgentMinVersion

IMO, the typical usage of datadog-go is to create once statsd with statsd.New + some options and never update it. If some users really care about using the latest features, REAME.md file and/or the release note can contain the recommended settings with their requirements.
IMO, using WithAgentMinVersion does not bring much compared to an up to date documentation (updating the version in the application using datadog-go or checking the documentation is the same amount of work).

If we really want to handle the Agent version, the Agent can expose the version as an endpoint. As there is a single feature using the Agent version, I think it does not worth adding this kind of endpoint at the moment.

[ogaca-dd]

LGTM.