V2: Support Environments in Evaluation and Analytics

[markphelps]

Now that we have decided to make environments a first class thing in v2, we need a way to support scoping evaluations (and metrics that are captured) to a specific environment.

We want to maintain backward compatibility as much as possible with the current v1 evaluation and OFREP APIs.

One thought is we would allow operators to define a 'default' environment that will be used for evaluations and also give them the opportunity to add an environment key in the header of the evaluation requests to override this default and support arbitrary environments in the request.

This would mean we would need to update all of our SDKs (both client and server side) to make setting this environment header easier for the caller.

For analytics, we want to capture the environment for all requests and add the environment in both ClickHouse and Prometheus 'backends'