[Nexthop][fboss2-dev] Add show running-config CLI command

[benoit-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Add a new fboss2 show command that retrieves the running configuration
from the agent via the getRunningConfig() Thrift API and displays it
as nicely formatted JSON.

This is useful when the config file on disk is missing or out of sync
with the running agent configuration.

Note: this change is part of a series, the previous one is #837, the next one is #843.

Test Plan

Added unit test CmdShowRunningConfigTest that mocks the getRunningConfig
thrift API.

Sample usage

[admin@fboss101 ~]$ fboss2 show running-config
{
  "sw": {
    "version": 0,
    "ports": [
      {
        "logicalID": 1,
        "speed": "HUNDREDG",
        "name": "eth1/1/1",
        ...
      }
    ],
    ...
  }
}