add new @login endpoint to return available external login options

[erral]

This PR adds a generic GET /@login endpoint that would expose links to additional login providers in a Plone site.

This way the endpoints added in packages like pas.plugins.authomatic and pas.plugins.oidc could be exchanged with adapter registrations and allow both products to be installed in the same Plone site for its use.

I am open to discuss whether the adapter needs to adapt just the IPloneSiteRoot object or perhaps may be a multiadapter including the request (perhaps to register the adapter for a given IThemeSpecific interface) or any other things.

[mister-roboto]

@erral thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

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[erral]

@jenkins-plone-org please run jobs

[erral]

This needs tests and documentation, similar to other endpoints. https://6.docs.plone.org/plone.restapi/docs/source/endpoints/index.html

Yes, I am preparing that.

[erral]

@jenkins-plone-org please run jobs

[erral]

I decided it would be faster, easier, and less frustrating to make the changes directly instead of a proper review. I hope that is OK.

Thanks! :heart:

[erral]

I am investigating why the output for the documentation is empty.

[stevepiercy]

@erral what output? The preview looks good: https://deploy-preview-1757--plone-restapi.netlify.app/endpoints/login.html

[erral]

@stevepiercy the last output, this one:

HTTP/1.1 200 OK
Content-Type: application/json

{
    "options": []
}

That should not be empty :/

[stevepiercy]

It's not an issue with the docs, because the docs only pull from this file:

https://github.com/plone/plone.restapi/blob/erral-login-options/src/plone/restapi/tests/http-examples/external_authentication_links.resp

Can you show what the content of that file should be?

[erral]

It's not an issue with the docs, because the docs only pull from this file:

https://github.com/plone/plone.restapi/blob/erral-login-options/src/plone/restapi/tests/http-examples/external_authentication_links.resp

Can you show what the content of that file should be?

I have already fixed this. There was a bug in my code :smiling_imp:

[erral]

@jenkins-plone-org please run jobs

[erral]

@jenkins-plone-org please run jobs