src.ctf.fs: implement `MetadataStreamDecoder` class

src.ctf.fs: implement `MetadataStreamDecoder` class

This class offers a `decode()` method that decodes a potentially
packetized metadata stream as a `std::string`.

After decoding a buffer, the `pktInfo()` method may return packet
information of the packetized metadata stream.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Iee867271fe6493b19c4bd77151409886b7c01ed9
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8050
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12257