Add options for user to use custom iofs path

[marvin659]

This change adds options for user to use custom iofs path and updates default iofs path to use container named volume.

Named volume works well for both Windows and MacOS so let use it by default to host the Fn listener socket file.

P.S.

• In my test on Windows, using OS path (windows host path) as iofs path does not work.
• For MacOS (with Podman or Rancher), there is an issue that the socket file could not be created under OS Path. See: [fnserver] fn invoke myapp hello on Windows: Error invoking function. status: 502 message: Container failed to initialize fn#1577 (comment)

This change fixes the following issue:

• [fnserver] fn invoke myapp hello on Windows: Error invoking function. status: 502 message: Container failed to initialize fn#1577