encoding/json: marshaller does not provide Base64Url support

[cyberphone]

1. Go version: 5.3
2. Operating system: Windows 8.1 64-bit
3. What did you do?
   https://play.golang.org/p/k1Wgec8cVB
4. What did you expect to see?
   I had hoped that there would be a field option allowing you to override the default encoding because Base64Url is nowadays the preferred format in standards including IETF's JOSE
5. What did you see instead?
   No such option :-)

[OneOfOne]

What's wrong with doing it client side instead of making the json encoder do mime sniffing on the data?

img.src = 'data:image/png;base64,' + data.Blob;

[cyberphone]

@OneOfOne There's nothing wrong with sniffing in a browser scenario but I'm targeting another class of applications where there is no room for such measures: https://cyberphone.github.io/openkeystore/resources/docs/jsonsignatures.html

Yes, an option must also work for unmarshalling.

[OneOfOne]

It's rather trivial to implement your own type for that though.

https://play.golang.org/p/FYFtvrdWnq

[cyberphone]

@OneOfOne Indeed I could but you should consider this issue related to a bunch of issues which makes the current Marshaller/Unmarshaller close to useless in my context:
#14750
#14749
#14135

[seankhliao]

see #63397 which proposes a struct tag of format base64url