Feat/rest api and file provider

[toddbaert]

This PR adds:

• generation of an OpenFeature-compliant REST API from a OpenAPI spec including models, validation, and parsing.
• a "evaluator" concept and package, with an IEvaluator interface. This is the "brain" doing flag evaluation. It knows hot to take a string from the ISync's Fetch method, and convert it to a particular schema, and then perform flag evaluations. The only implementation so far is the JsonEvaluator, which uses the schema here (included as a submodule)

The Chi framework was used since it has no dependencies, and is compatible with native go net/http handlers.

To test:

• pull in the schema submodule with git submodule update --init --recursive
• go mod download
• run make build (which is now includes of make generate). The generated code is .gitignored.

Then, you can perform flag evaluations after running ./flagd start --uri ./examples/provider_flags.json

curl -X POST localhost:8080/flags/myBoolTest/resolve/boolean?default-value=true
// {"reason":"DEFAULT","value":true}

curl -X POST localhost:8080/flags/myStringTest/resolve/string?default-value=hi
// {"reason":"DEFAULT","value":"red"}

curl -X POST localhost:8080/flags/myNumericTest/resolve/number?default-value=123
// {"reason":"DEFAULT","value":1}

curl -X POST localhost:8080/flags/myObjectTest/resolve/object?default-value=foo,bar
// {"reason":"DEFAULT","value":{"color":"blue"}}

Closes: #23
Closes: #27

[AlexsJones]

Hey great stuff, Can we talk this one through on a zoom call @toddbaert ?