Allow to add ValidationOptions to NewRouter()

[yclybouw]

In routers/legacy/route, we have the function:

func NewRouter(doc *openapi3.T) (routers.Router, error)

Which calls:

doc.Validate(context.Background())

As doc.Validate accepts validation options (signature: func (doc *T) Validate(ctx context.Context, opts ...ValidationOption) error) I would like to modify the NewRouter function (or add a new one named NewRouterWithValidationOptions), allowing to provide validation option:

func NewRouter(doc *openapi3.T, opts ...ValidationOption) (routers.Router, error)

If this is fine for you, I can create a pull request for it.