Add support for OneLogin

[pavelnikolov]

This PR adds OneLogin provider support. There are few things that would like to discuss. OneLogin is a bit different than other providers. In order to access their API, each application needs to have app_id, client_id, client_secret. Without these I cannot generate access_token, which is in turn used to generate SAML assertion and to verify MFA options.

This PR is very naive and adds the app_id into the common configuration and the when prompting for credentials I check if the provider is OneLogin and I validate if client_id and client_secret are provided. Ideally, I would like to refactor the library (i.e. change the signature of the providers) to allow each provider to request/prompt for additional configuration options and keep the common clean. Any feedback and comments would be highly appreciated.

Unit tests will be ready very soon.

[pavelnikolov]

Ideally, I don't want to put these 4 new fields there. They are only needed for OneLogin. Any ideas where to put them? I wanted to refactor the library and allow each provider to have custom config and secrets. Such change would make this PR huge so I would split changes and refactoring in different PRs.

[pavelnikolov]

This PR solves #121

[wolfeidau]

So as it turns out I am going to add those attributes for another provider soon to support openid based login.

Although this is a big change and you have touched a bunch of things I do love getting new providers so happy to take it as is.

Cheers for the work you have done!

[btsteve]

Is there an example of what need to go in each field. For Subdomain am i adding just the host part or the full url? Where do i get the client_id, secret, and app id

[pavelnikolov]

Subdomain is your organisation unique name. It is usually the subdomain part of the One Login dashboard URL.

Please, follow this tutorial to generate client credentials https://developers.onelogin.com/api-docs/1/getting-started/working-with-api-credentials