Configuration file support

[michaelklishin]

rabbitmqadmin v1 can use a TOML/ini-style configuration file for setting command defaults.

This is not a commonly used feature but it would be nice to support it for ease of migration.

A few nuances worth considering are mentioned in this discussion. The ClapSerde library mentioned there is AGPLv3-licensed, so we cannot adopt it.

Loading the file very early on to populate CLI parser defaults is my favorite option at the time of writing.