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

Providers

• #119 - Remove context transformers. Add provider hooks #140

Evaluation Context

• #117 - Ensure context merging behavior is correct #141
• #120 - Add the ability to fetch all custom fields
• #121 - Ensure evaluation context keys are unique

Hooks

• #119 - Ensure provider hook are correctly ordered #142

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

[toddbaert]

120 - I believe is "free" in JS... we can use Object.keys I think.
121 - I think this is more or less "for free" too, and we already freeze the object after the before hooks.

119 - seems like it will be done with the testing of #140

Let me know what you think @beeme1mr