Add MVP support for SLP2 tokens.

Note that, since Cashtab uses ecash-lib, which already supports SLP2, there is not much difference between SLP1 and SLP2 for these methods. Because the mint spec is different, implementing genesis and minting is slightly more involved. We could do it here but imo better to get this in so we have support as soon as possible.

npm test

see test site at https://cashtab-musd-test.netlify.app/

I can send you some test tokens if you need them