cpp-common: add `readFixedLenInt()` function

cpp-common: add `readFixedLenInt()` function

This function template reads an integer from a buffer without assuming
its byte order.

For example, this is needed when reading the magic number from a data
buffer of unknown byte order. If needed, the user then has to manually
swap byte order of that integer to get its native byte order

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ia94ee3a3ffc7caaa46a46defead3e23da0a4c2b2
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8111
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10841
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>