(feature) Introduce concept of flag set

[thomaspoignant]

Requirements

As of today, GO Feature Flag is evaluating flag differently for client and server evaluations.

• In the client-side, we are evaluating the flags in bulk, and we are keeping the evaluation responses in memory for the duration of the application.
• In the server-side, we are evaluating the flags 1 by 1, and we are not keeping the evaluation responses in memory (except if a cache is configured).

In both cases having a flag set could be beneficial, for performance reasons, but also for organization reasons.

[thomaspoignant]

A PR is open for the specification #2734.
It would be also great to be able to generate flag sets automatically as mentioned in #2314.