Pack base64 as a feature

[DanGould]

right now you need an external base64 dep, either on its own or typically exported from bitcoin. Since we already use it in the lib we could re-export it or make a helper to get PSBTs in base64 for transit.

[DanGould]

packed with bitcoin = { features = [ "base64"] } in 0.9.0 but not exported as its own feature. Export it!