The new argument allows the user to pass a JSON of properties to merge
into the generated launch.json config when setting up vscode-lldb
forwarding. This way the user can add pre-build tasks, extra init
commands etc.
Test: atest gdbclient_test
Test: lldbclient.py --setup-forwarding=vscode-lldb
--vscode-launch-props='{"sourceMap": {"test1": "test2"},
"postDebugTask": "Stop LLDB client", "processCreateCommands" :
["test"]}' -r test
Change-Id: I763dd15dde10421e86bc0a6ddfde974156ef1588
