src/cpp-common/bt2: add field location support

src/cpp-common/bt2: add field location support

This patch adds the `bt2::ConstFieldLocation` wrapper class as well as
bt2::TraceClass::createFieldLocation().

I didn't implement a `bt2::CommonFieldLocation` template class because a
field location is immutable anyway, so we only need the const version.
bt2::TraceClass::createFieldLocation() returns a shared const field
location.

bt2::TraceClass::createFieldLocation() accepts a span of strings (either
`const char * const` or `const std::string`) to make the interface a
little nicer than a raw array of `const char *` and a matching size.
Creating a field location is a slow-path operation anyway.

Also adding bt2::wrap() accepting `const bt_field_location *`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I160052c5a787114b52062aa87908c94a2af40f32
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8016
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12699