Rework receive::v2 submodule into additive crate::v2 and crate::v1 submodules features

[DanGould]

I don't really love how the features are laid out inside receive especially. Send is not as big of a deal because the state machine has been flattened. If there were separate modules for separate features I think they'd be easier to reason about from docs.rs

Consider removing the not(feature = v2) feature since we want to discourage its use and one can replicate v1 soverignty by running both a directory and payjoin client on the same machine. Rust features should be additive.