add support for additional salt.auth types

[erwindon]

In #367, @hoaivan requests the addition of auth method sharedsecred.
However, that is not a safe mechanism unless additional measures are taken to prevent leaking these credentials.
Therefore sharedsecret will not be added as a standard option in the login screen.

But here is an alternative...
A file salt-auth.txt can be placed on the server, which contains all of the additional auth methods that should be shown.
So additional options will only be visible when they are added to that file.
And also, any other auth mechanism can be made selectable.

Note that an auth method does not work unless it is also configured in the master file.

This PR replaces the original PR #367.

[erwindon]

@hoaivan can you please give the code from this PR a try?

[erwindon]

@hoaivan I did not receive a reaction. can you still please try this PR?

[hoaivan]

I will schedule my time for this.

[erwindon]

@hoaivan do you have some time in the coming days? If not, I'll just trust my own testing and merge this PR.

[erwindon]

@hoaivan it seems you are unavailable.

I've tested the updated login screen and:

• it works properly with salt-auth.txt absent
• it works properly with salt-auth.txt as an empty file
• it works properly with salt-auth.txt containing only sharedsecret
• it works properly with salt-auth.txt containing sharedsecret and xyz (though choosing xyz leads to nothing)