Simplify file handling in usage of pyxform

Since pyxform 2.1.0 (released Sep 2024) it is possible to exchange bytes with `xls2xform.convert`, instead of the pattern pyxform-http currently uses of writing out to a file for `xls2xform.xls2xform_convert` then reading the result file(s) to send back - [as seen here](https://github.com/getodk/pyxform-http/blob/82825166d0ec9c5fd0ea577da868f15bb6285e42/app/main.py#L45-L51). If ODK validate is run (which it currently is by default), pyxform has to write out the XForm XML for that, at least until odk validate can accept data via stdin https://github.com/getodk/validate/issues/97. If both of these items are resolved then pyxform-http would not need filesystem write access.

Start in app/main.py at the file-handling pattern around lines 45-51, then review pyxform 2.1.0's xls2xform.convert byte exchange and ODK Validate issue #97. The work is complete when pyxform-http no longer needs filesystem write access, subject to both validation and conversion paths supporting in-memory data.

Written by the indexing model from the issue text.