Introduce the SDK Evaluation Context object

## Abstract

While implementing #4278, a [discussion](https://github.com/Flagsmith/flagsmith-go-client/pull/133#discussion_r1694892206) has occurred about how we should approach SDK changes in the future.

The consensus is we should take after OpenFeature and implement a common _evaluation context object_ that a single `getFlags` interface should accept as its only input.

I'm creating this issue as a hub for further implementation discussion and planning.

## Schema proposal

You can observe and review the proposed schema in #4414.

## Implementation plan

For implementation phase 1, We should settle on one of the following:
- Implement it in the Golang SDK as part of #4278, and evolve the existing interfaces for the rest of the SDKs.
- Implement it for all SDKs as part of #4278.

For phase 2, we should consider a single `/api/v2/flags` API with a schema identical to the Evaluation Context schema. Should we go forward with this, it'll allow us to fully code generate remote evaluation parts of the SDKs!