Meson option to build without libchamplain

[meisenzahl]

Using libsoup2 and libsoup3 in the same process is not supported. On distributions with more recent dependencies, libchamplain still uses libsoup2 as dependency. Therefore, Calendar crashes immediately after startup.

$ io.elementary.calendar 

(process:1855): libsoup-ERROR **: 06:31:48.760: libsoup2 symbols detected. Using libsoup2 and libsoup3 in the same process is not supported.
Trace/breakpoint trap (core dumped)

This PR adds a flag to disable libchamplain as a dependency. Tested on Fedora Rawhide.

With libchamplain

Without libchamplain

@tintou has provided a patch for libchamplain that allows to use libchamplain with libsoup3 libraries and applications. If a distribution cannot use this patch, we can provide an alternative with this PR.

[meisenzahl]

Closing in favor of https://github.com/elementary/calendar/pull/787