feat: allow the auth context to be configured

[qraynaud]

This is useful because ADFS does not recognize the default value in this plugin (urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport) and wants http://schemas.microsoft.com/ws/2008/06/identity/authenticationmethod/windows in there instead.

I believe you can also set disableRequestedAuthnContext to true to get this to work. Since this parameter was not documented I also added an entry for it.

[ploer]

Thanks for the PR, looks good!

Two requests:

1. Let's call the option authnContext instead of authenticationMethod, since that's the underlying SAML name.

2. Please add a mocha self-test for this behavior. (probably an additional entry in the 'captured SAML requests' group)

[qraynaud]

Thanks for the quick review. I'll make the suggested modifications early next week.

[qraynaud]

done, this should be better

[ploer]

Looks good, thanks!