Advertise the appropriate input mechanisms for credentials

[leonardr]

This is one of a series of proposals for taking NYPL's extensions to Authentication For OPDS and adding them to the core document.

https://github.com/NYPL-Simplified/Simplified/wiki/Authentication-For-OPDS-Extensions#input-mechanisms

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An OPDS server may use the inputs extension to customize the input mechanisms used to gather the patron's credentials.

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Of all the proposals this is IMO the most useful one to add to the core spec.

[HadrienGardeur]

I agree that this would be a useful addition to the spec as well.

For the length of each field, I think that instead of just having a max value we might want to support the following:

• min
• max
• exact

We'll of course need rules when these values contradict one another, but not having min or a specific value feels like a missed opportunity.

For the keyboard, what you've defined is a good starting point, but I'd rather have lowercase tokens without spaces:

• email
• number
• none

This would further align our input types with HTML. Do we really need default when having a null value is enough?

The barcode format feels a little under specified currently with a single value. This might be more suited as an extension.