Define an API schema for flagsmith SDKs to send evaluation data

[matthewelwell]

The goal of this issue is to define the API endpoint schema that the SDKs will send data to based on the evaluations that occur on the client side. Bear in mind that this is an alpha release, WE DO EXPECT TO HAVE TO BREAK THIS SCHEMA IN THE FUTURE.

The data that we want to include in this schema are:

• name of the feature that was evaluated
• enabled state of the feature evaluated
• value of the feature evaluated
• timestamp of the evaluation
• identity that evaluated the feature
• traits of the identity that evaluated the feature
• list of segment names for the identity

Question: One thing to consider is the encapsulation of the data for this endpoint - should this endpoint consume a single atomic evaluation only, or a bulk number of actions compiled by the SDK over a period of time?
Answer: The expectation is that it will consume 'batches' of evaluations, not individual evaluations.

Note 1: we should be able to collect the SDK language / version from the user-agent header which we will need for the schema to the big data platforms.

Note 2: we have an existing endpoint here from a previous attempt based on work around split testing. This may or may not be relevant / helpful here.