vscode: Add configurations to run the executables under the debugger

Add tasks.json and launch.json which allow VSCode users to build the
project and run the various binaries (lttng, lttng-relayd,
lttng-sessiond) under the integrated debugger.

For the moment, the configuration assumes the user wants to build
"in-tree" and has setup the tree to build the project (running
./bootstrap and ./configure).

The build job attempts to build a compile database if 'bear' is
available on the system.

To debug the LTTng client, make sure to edit the matching configuration
in .vscode/launch.json to provide your desired arguments (for the
moment, 'help' is passed by default).

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Iee6d6e012bef82f5d3df4296925a3669ad7027d6