Rework `receive::v2`, `send::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.

• separate send::{v1, v2} modules Abstract send feature error variants into separate feature-specific abstractions #464
• Fix PjUriBuilder feature mess Separate receive submodules to be additive #466
• Place receive v1 typestate machine into receive::v1 module with feature-gated visibility
• Separate receive::{v1, v2} errors (related: Review usage of crate::receive::Error #312,) Separate receive::{v1,v2} error modules #482
• set optional_parameters::SUPPORTED_VERSIONS according to calling module, not feature flag #490

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follow ups:
#500

[DanGould]

@nothingmuch I believe making PjUriBuilder::new into variants rather than a feature-gated set of arguments with the same method, perhaps v1::PjUriBuilder / v2::PjUriBuilder if it can't be entirely contained in the receiver state machine? is the last remaining TODO item to make the v1/v2 features additive after #464. What's the status of that refactor?

[nothingmuch]

although i initially started with PjUriBuilder due to #416, i it's way up my "call stack", as it were, feel free to proceed with such changes, at worst it will be a minor disruption for my changes to payjoin-cli