Add a CLI command to list and get a value from dask config

[j-bennet]

Adds two cli commands, git config get to get a value from dask config:

> dask config get distributed.scheduler.dashboard
bokeh-application:
  allow_websocket_origin:
  - '*'
  check_unused_sessions_milliseconds: 500
  keep_alive_milliseconds: 500
status:
  task-stream-length: 1000
tasks:
  task-stream-length: 100000
tls:
  ca-file: null
  cert: null
  key: null

or git config list to print the whole config:

> dask config list
array:
  backend: numpy
  slicing:
    split-large-chunks: null
  svg:
    size: 120
coiled:
...

Partially implements #9746.

• Tests added / passed
• Passes pre-commit run --all-files

[j-bennet]

cc @ntabris

[milesgranger]

Just one nit/question, but I don't think my approval truly means anything yet without more permissions. :)

[hendrikmakait]

Thanks, @j-bennet. I have one minor nit; feel free to ignore it.