Add `ag device list` command to list and query device plugin states

## Motivation  

Currently the compute device plugins generate log messages about their device detection results only at the agent startup. Sometimes in customer sites, we need to inspect how they are recognized after a long time since startup, when the log messages may be lost due to rotation.

## Required Features

- Agent CLI command(s) to list and query the detected compute devices and their capacity, just like the log messages shown during the agent startup.
- Agent CLI command to perform the fraction ratio conversion to show “how much `xxx.shares` corresponds to a designated accelerator device” if the designated device plugin supports the fractional allocation mode.

## Testing Scenarios  

- Use the dummy device plugin to simulate discrete/fractional devices.

JIRA Issue: BA-1065