Add support for unknown messages

We do retain some template instantiations by default, but only for types
actually used in the public API. If anyone wants to use the other ones,
they will need to import the "*_impl.hpp" defining their respective from
and to json functions. Everyone else should just try to parse only the
types we use in our variants and such.

fixes #93
fixes #80

This is partially based on the report and patch from
Jason Volk <jason@zemos.net>, however it actually fixes the issue
properly in both cases and doesn't rethrow the callbacks own exception
for error handling... It also improves the error message for aborted
callbacks by hardcoding the curl error code. This regularly happens when
you close Nheko and it cancels the sync request.

Most password managers used 100k until recently for the weaker sha256
version. We used 500k for the sha512 variant, so we should be good for a
while. Hopefully Matrix switches to argon2 or something better at that
point.

Fixed build under GCC 13

Signed-off-by: Vitaly Zaitsev <vitaly@easycoding.org>

It makes no sense for servers to send the parent object without events
and not sending an empty list, but strictly speaking that is spec
compliant, so...

Relates to https://github.com/Nheko-Reborn/nheko/issues/1257

add user directory search

Fix template instantiation for Confetti