Provide plugins a way to read/write files on remote hosts

[mothslaw]

Is your feature request related to a problem? Please describe.
Sometimes a plugin wants to read or write a file (e.g. a DMBS configuration file). In order to do this today, they have to construct a bash script and use stdin/stdout to transfer the file contents back and forth.

Describe the solution you'd like
We could provide functions libs.read_file(connection, path) and libs.write_file(connection, path, file_contents) to make this easier for the plugin.

Additional context
As with any new libs function, this will require new engine functionality. However, the communcations layer in the engine already does support this, so most of the hard work is done. We'd just have to poke this through to the plugin layer.