htrun: Request to add a ConnectorPrimitive base on files/stdio

[LDong-Arm]

Currently, htrun supports communicating with a device-under-test (i.e. one that runs greentea-client) in one of the following ways:

• serial port
• remote (e.g. RaaS)
• Fast Models (via a socket)

But if we want to the whole test setup (device & host) to be on a PC only, which is useful for integration testing for example, support for stdio/files by htrun is valuable in allowing the "device-side" program and htrun to communicate as two processes.

To implement this, add a class of ConnectorPrimitive and command line options for passing input/output files.