box: refactor net.box response body decoding

Response body decoding of DML and call/eval requests is very ad-hoc and
hard to extend: introduce a new `response_body_decode` helper that decodes
the response body similarly to `xrow_decode_dml` — this will allow to
separate decoding from processing.

Needed for #8147

NO_CHANGELOG=refactoring
NO_DOC=refactoring
NO_TEST=refactoring

(cherry picked from commit 1d6043fa)