fully implement support for the `vRun` gdb packet

[andrurogerz]

All of the plumbing is in place in DS2 to support the vRun packet, but somehow neither platform nor debug session implementations actually implement SessionDelegate::onRunAttach.

Per the gdb packet documentation:

vRun;filename[;argument]…
Run the program filename, passing it each argument on its command line. The file and arguments are hex-encoded strings. If filename is an empty string, the stub may use a default program (e.g. the last program run). The program is created in the stopped state.
Reply:
Any stop packet

These 4 lldb test cases fail against DS2 because vRun always fails:

GdbRemoteLaunchTestCase.test_QEnvironmentHexEncoded_llgs
GdbRemoteLaunchTestCase.test_QEnvironment_llgs
GdbRemoteLaunchTestCase.test_launch_via_vRun_llgs
GdbRemoteLaunchTestCase.test_launch_via_vRun_no_args_llgs