feat: add config profiles to llamactl

[masci]

This PR adds support for config profiles to llamactl. From now on, llamactl will store a configuration file with the following format:

default:
  insecure: true
  server: http://localhost:4501
  timeout: 10.0

prod:
  insecure: false
  server: http://example.com:8080
  timeout: 120.0

Users can specify what profile to use by passing its name with the option --profile. The default profile will be selected when --profile is not used.

[coveralls]

coverage: 81.085% (+0.3%) from 80.818%
when pulling c1a581d on massi/config-profiles
into 05e05f9 on main.