[lldb-dap] Add a CMake variable for defining a welcome message (#78811)

lldb-dap instances managed by other extensions benefit from having a
welcome message with, for example, a basic user guide or a
troubleshooting message.
This PR adds a cmake variable for defining such message in a simple way.
This message appears upon initialization but before initCommands are
executed, as they might cause a failure and prevent the message from
being displayed.