Add OpenAPI entry-point for the go-provider-sdk

[iwahbe]

Add a new provider entry-point to allow consuming OpenAPI schemas.

Defining a provider currently looks like this:

package main

import (
	_ "embed"

	schema "github.com/getkin/kin-openapi/openapi3"
	p "github.com/pulumi/pulumi-go-provider"
	s "github.com/pulumi/pulumi-go-provider/middleware/schema"
	"github.com/pulumi/pulumi-go-provider/openapi"
	"github.com/pulumi/pulumi/sdk/v3/go/common/util/contract"
)

//go:embed test-api/openapi.json
var TODOSchemaBytes []byte

func main() {
	doc, err := schema.NewLoader().LoadFromData(TODOSchemaBytes)
	contract.AssertNoError(err)
	m := openapi.New(doc)

	resources := []openapi.Resource{{
		Token:  "todo:index:Todo",
		Create: m.NewOperation(doc.Paths["/todos"].Post),
		Read:   m.NewOperation(doc.Paths["/todos/{todoId}"].Get),
		Update: m.NewOperation(doc.Paths["/todos/{todoId}"].Patch),
		Delete: m.NewOperation(doc.Paths["/todos/{todoId}"].Delete),

		Mappings: openapi.Mappings{
			openapi.MapCreate("id").To(openapi.MapAll("todoId")),
		},
	}}
	err = p.RunProvider("todo", "0.1.0", openapi.Provider(openapi.Options{
		Metadata: s.Metadata{
			Description: "A example provider of a todo app",
			DisplayName: "Todo",
			Homepage:    "https://h88namdrg9.execute-api.us-west-2.amazonaws.com/stage/",
			Publisher:   "The Demo Gods",
		},
		Resources: resources,
	}))
	if err != nil {
		panic(err)
	}
}

[iwahbe]

Closing in favor of #213.