[lib] Introduce SIWEMessage type

Summary:
Effectively copied from the TypeScript types:

Will be helpful to type return value of new SiweMessage(message) in siweAuthResponder as SIWEMessage.

Depends on D6029

Test Plan: Careful reading

Reviewers: tomek, ashoat

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6030