added `Basic Auth` authentication method

[flydev-fr]

I needed Basic Auth authentication method for a project.

Feel free to revise and merge it if you think it can be a cool addition to the module.

[brettwilcox]

Thank you for sharing this code! I also need basic authentication for my application (as it's an enterprise app that has other applications pulling information from processwire). This tied in nicely with LDAP https://modules.processwire.com/modules/ldap-sign-in/ and I was able to accomplish what I needed.