`ReadSource`, `WriteSink`, `BufferSource` and `Sink` should be returned to capy

[ashtum]

The changes made in #368 were the result of a miscommunication in a Slack conversation. @vinniefalco suggested moving the push_to/pull_from algorithms to the Boost.HTTP library, but instead, all the concepts and type-erased vocabulary types were moved to the Boost.HTTP library.
Related Slack thread: https://cpplang.slack.com/archives/C02MQKSSMH8/p1787988636975289

[akrzemi1]

Should we also pull push_to and pull_from from Http and push them back to Capy?