Update SDK to be compliant with spec v0.2.0

[beeme1mr]

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

Flag Evaluation API

• #116 - Have different methods for float and int - Feat: Implement seperate methods for integer and double #31

Providers

• #119 - Remove context transformers. Add provider hooks - Feat: Add support for provider hooks #38

Evaluation Context

• #117 - Ensure context merging behavior is correct - Chore: Evaluation Context must only contain unique values #32
• #120 - Add the ability to fetch all custom fields - Chore: Prove able to fetch all fields in EvaluationContext #37
• #121 - Ensure evaluation context keys are unique - Chore: Evaluation Context must only contain unique values #32

Hooks

• #119 - Ensure provider hook are correctly ordered - Feat: Add support for provider hooks #38

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

[toddbaert]

I think we'll be done when @benjiro 's PR is merged, and when I complete this: #38 (comment)

[benjiro]

Version 0.1.2 has been released which is Spec v0.2.0 compliant