Update SDK to be compliant with spec v0.5.0

[beeme1mr]

Update the SDK to be compliant with the v0.5.0 spec.

What's Changed

⚠ BREAKING CHANGES

• feat!: make error code an enum and include optional error message spec#142 Error codes must be a constrained set of values (e.g. enum).

Additional changes

• add reasons, table spec#140 Clarify that reason must be a string in the flag resolution details and evaluation details.
• feat!: make error code an enum and include optional error message spec#142 Add optional error message to flag resolution details.

General

• Add/update a spec version badge on the readme.

Full Changelog: open-feature/spec@v0.4.0...v0.5.0