Update SDK to be compliant with spec v0.2.0

[beeme1mr]

Update the SDK to be compliant with the v0.2.0 spec.

• Determine correct context merge order #43

Flag Evaluation API

• #116 - Have different methods for float and int

Providers

• #119 - Remove context transformers. Add provider hooks

Evaluation Context

• #117 - Ensure context merging behavior is correct
• #120 - Add the ability to fetch all custom fields
• #121 - Ensure evaluation context keys are unique

Hooks

• #119 - Ensure provider hook are correctly ordered

Full Changelog: open-feature/spec@v0.1.0...v0.2.0

[justinabrahms]

Still remaining:

In the spec, but not in our tests: 
  4.4.7: If an error occurs in the before hooks, the default value MUST be returned.
  3.2.1: The API, Client and invocation MUST have a method for supplying evaluation context.
  3.2.2: Evaluation context MUST be merged in the order: API (global) - client - invocation, with duplicate values being overwritten.
  3.1.4: The evaluation context fields MUST have an unique key.

[justinabrahms]

Done. We'll call this ticket done when #43 is resolved.

[beeme1mr]

Great, thanks Justin.

[beeme1mr]

Ah, sorry. I misread your comment. I'll leave it alone 😃