Support optional scheme being provided in msg header

[NicholasEllul]

Context

The EIP-4361 specification outlines the abnf message format that corresponds to a valid SiWE message. As seen below, this format indicates that there is an optional scheme that can pe provided alongside the domain in the first part of the message

sign-in-with-ethereum =
    [ scheme "://" ] domain %s" wants you to sign in with your Ethereum account:" LF
    ...

Problem

The siwe-parser currently fails when a scheme is provided alongside the domain. This is due to the fact that our abnf grammar string currently lacks this [ scheme "://" ] declaration in the grammar, resulting in valid SiWE messages containing this scheme to result in an error being thrown.

Solution

This pull request does two things to solve this:

• Adds [ scheme "://" ] to the grammar string
• Adds logic to parse the scheme from a given message
• Add support for the optional scheme being provided in a SiweMessage
• Adds a test case containing the example containing a scheme that is specified in the EIP-4361 spec. This will ensure we do not have any regressions in the future.

[sbihel]

Thank you for the PR! Would you mind adding support for the other packages as well?

[NicholasEllul]

@sbihel thanks for the callout - I've now made the respective changes

[sbihel]

I'm on the fence to have scheme be part of or separate from domain. But it think this way is a more natural addition

[sbihel]

Published as part of @spruceid/siwe-parser@2.1.0 and siwe@2.2.0.