Add Base45 operations

[t-8ch]

Closes #1219

[CLAassistant]

All committers have signed the CLA.

[nielsbasjes]

I was looking into decoding the vaccination QR code (which uses Base45) and ran into this tweet which seems to indicate that there are variants of this encoding: https://mobile.twitter.com/jilles_com/status/1416758126372663305

It would be nice if these variants can be supported too.

[t-8ch]

@nielsbasjes The EU specification does not specify any variants. Are they documented somewhere properly?

[t-8ch]

FYI to properly decode the EU green pass files CyberChef will also need support for COSE

[nielsbasjes]

Ik have also not been able to find a good specification yet.
The best sources I have are the tweet I mentioned which shows a code fragment and this (Dutch) article which also mentions ASN.1 as being part of the Dutch variant.
https://tweakers.net/reviews/9166/het-digitale-coronapaspoort-hoe-de-app-om-te-kunnen-reizen-werkt.html

If I find something better I'll post it here.

[nielsbasjes]

It seems there are several publicly available photos that seem to actually show a valid (expired) Dutch QR code:
https://twitter.com/MinVWS/status/1438433503205597192/photo/1
https://www.ewmagazine.nl/nederland/achtergrond/2021/09/radicale-omslag-coronabeleid-toegangsbewijs-voor-horeca-en-theater-geen-afstandsregel-844321/

Note there are also a lot of images that have fake content. These two seem to contain (at a first glance) content that makes sense.

[nielsbasjes]

The code for the Dutch apps was opensourced by the Dutch government: https://github.com/minvws/nl-covid19-coronacheck-app-coordination

[t-8ch]

This seems to be their base 45 implementation:
https://github.com/minvws/base45-go

The rest of their implementation is in https://github.com/minvws/nl-covid19-coronacheck-mobile-core and https://github.com/minvws/nl-covid19-coronacheck-idemix

A few notes:

• I think this is a completely different encoding than the one from this PR, so it probably should get its own dedicated operation.
• There is no public specification, so you would have to work from the code which is licensed as EUPL, for which I'm not sure if it's compatible with the CyberChef license.

[nielsbasjes]

I fully agree.
This dutch encoding must be treated completely separate from this merge request.