src/cpp-common: add {fmt} 10.1.1

src/cpp-common: add {fmt} 10.1.1

This very famous library [1] should be useful to format all sorts of
strings in the future instead of using `std::string` or
`std::ostringstream`, as well as to implement a C++ logger.

We're building `format.cc` and `os.cc` which should speed up the rest of
the build and maybe reduce the build size (not using `FMT_HEADER_ONLY`).

Adding `.reuse/dep5` to provide licensing information for all the source
files in `src/cpp-common/vendor/fmt` without modifying the original
ones.

[1]: https://fmt.dev/

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I65cfbca05e5481847ce7183a2a2b20c25ae260ad
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11387
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>