Sign-in with stacks

[friedger]

This PR defines a standard for message used for sign-in with stacks (#69)

It is mainly inspired by EIP-4361.

The standard requires the following limits that can be discussed:

• URLs are limited to 80 ascii characters so that they can be shown on a normal screen.
• 10 resources can be added to the message.

[obycode]

I'd love to see this revived. Has anyone built any implementations of this?

[pradel]

@obycode I have a working implementation for Sigle, if you are interested in using it I can publish it on npm. Can extract it to a smaller repo and publish it there https://github.com/sigle/sigle/tree/main/packages/sign-in-with-stacks

[pradel]

Published it as a separate package on npm npm i sign-in-with-stacks and github at https://github.com/pradel/sign-in-with-stacks for anyone to use!