go: vendor the oapi-codegen cmd

See the comment in tools.go, I cannot fully explain what's happening here.
Somehow, Go 1.14 wants to use the vendored version of oapi-codegen but
without this file, oapi-codegen isn't vendored so the generation fails.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

vendor/github.com/getkin/kin-openapi/openapi3/security_requirements.go

@@ -0,0 +1,43 @@
+package openapi3
+
+import (
+	"context"
+)
+
+type SecurityRequirements []SecurityRequirement
+
+func NewSecurityRequirements() *SecurityRequirements {
+	return &SecurityRequirements{}
+}
+
+func (srs *SecurityRequirements) With(securityRequirement SecurityRequirement) *SecurityRequirements {
+	*srs = append(*srs, securityRequirement)
+	return srs
+}
+
+func (srs SecurityRequirements) Validate(c context.Context) error {
+	for _, item := range srs {
+		if err := item.Validate(c); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+type SecurityRequirement map[string][]string
+
+func NewSecurityRequirement() SecurityRequirement {
+	return make(SecurityRequirement)
+}
+
+func (security SecurityRequirement) Authenticate(provider string, scopes ...string) SecurityRequirement {
+	if len(scopes) == 0 {
+		scopes = []string{} // Forces the variable to be encoded as an array instead of null
+	}
+	security[provider] = scopes
+	return security
+}
+
+func (security SecurityRequirement) Validate(c context.Context) error {
+	return nil
+}