Add vendor/fmt

Add fmt 8.1.1 headers (we will use it in header-only mode). fmt is made
available under the MIT license, which is already in the LICENSES
directory.

Note that an lttng-format.hpp header is added to disable a warning which
prevents us from building with -Werror.

../../../src/vendor/fmt/format-inl.h:2457:11: error: target of initialization might be a candidate for a format attribute [-Werror=suggest-attribute=format]
 2457 |     int (*snprintf_ptr)(char*, size_t, const char*, ...) = FMT_SNPRINTF;
      |           ^~~~~~~~~~~~

The header also ensures that FMT_HEADER_ONLY is defined for all uses of
libfmt.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I5696c09d6e07716b955091922bb27ce082fb2686