Add default defineEndpoint in base SaloonRequest

[ajthinking]

Just got started with this for a graphql integration, works really good 💫
Not 100% sure this is an improvement you would like so feel free to decline it 👍

Since graphql request always targets a root like "www.acme.com/api/graphql" - no appended endpoint - would it make sense to allow to not explicitly repeating defineEndpoint method returning '' every time?

    /**
     * Define the endpoint for the request.
     *
     * @return string
     */
    public function defineEndpoint(): string
    {
        return '';
    }

This PR adds the above to the base class as a default.

Also let me know if you prefer keeping all tests in function block style. I just discovered that higher order thing 😄

[Sammyjo20]

This is a great little improvement! 🙌 Glad it is working well for GraphQL

Thank you for the PR ❤️