Optional sequential number certificate code, with new display element.

[rickabitdigital]

Add a sequential number code element with an optional user defined start number setting.

• with option to include this sequential number and/or other element tokens in the certificate filename (will create separate ticket).
• also with option to download zip of issued (renamed) certificates in batches by Group ref: Add group name element #155 .

I've written the logic for this sequential number and start number setting outside of Moodle for easier testing.
I will now attempt to incorporate into plugin code.

@mdjnelson , @leonstr, @dpalou, @rhell4, @Peterburnett, @mwreichel, @mohamedmohamedatia, @ojnadjarm apologies if this is the wrong way to request feedback (please ask and I will delete your name) - I simply included committers from the last 6 months as most relevant interested parties.
I would really appreciate some feedback on how best to incorporate this feature given that customcert currently relies heavily on the existing random code generation. e.g. should I fork and overwrite, swapping random code for sequential? Or should I leave random code generation, and it's display element, and include a user defined option to allow either or both?
This refactor would be easier if file naming was tokenised to include all available display elements/fields as tokens.

[rickabitdigital]

I'm testing this feature now.