cpp-common/bt2c: add `UnicodeConv` class

cpp-common/bt2c: add `UnicodeConv` class

Add the `UnicodeConv` class, which currently implements conversion
from (UTF-16, UTF-32) × (BE, LE) to UTF-8.

The `UnicodeConv` class uses `g_iconv` from GLib internally to make
the conversions.  It has a vector member that is used as an output
buffer.  Public conversion methods accept a span of
`const std::uint8_t`, and return a span `std::uint8_t` which is a view
on that internal vector.

Add a test which:

✤ Converts some hardcoded UTF-16 and UTF-32 strings to UTF-8 and
  verifies the result against a reference UTF-8 string.

✤ Feeds the UTF-16 and UTF-32 conversion methods with truncated strings
  and verifies that they throw and append specific causes to the error
  of the current thread.

Change-Id: I962bd49261a3d9779ed6a24a26c7800a24beb719
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12715