[lldb-dap] lldb-dap server mode management improvements

[ashgti]

Filing this to circle back to improving the lldb-dap server mode management from the VSCode extension.

Some ideas that are not set in stone:

• The server could shut itself down if the process gets a low memory warning and there are no active clients.
• The server could have an idle timeout and shutdown if there are not active clients after some configurable period.
• The extension could alert the user to unexpected crashes or exits.
• The extension could have commands to manually start / stop the server.
• We could run one server per host instead of the default behavior of one per-window. Its possible to spawn the server in a different process group than the extension host process and advertise the connection parameters in a different way (e.g. the server can use a unix socket and place the socket in ~/.lldb-dap-sock then if the socket file exists we could use it across extension hosts).

[ashgti]

#129332 Implements a memory monitor on macOS and linux to trigger SBDebugger::MemoryPressureDetected() which should help by removing unreferenced symbols.