Replace toml configuration with CLI + env args

[Mirko-von-Leipzig]

Originally suggested in #647, paraphrasing motivation here.

toml files are nice for low-change, manual configuration as it gets out of the way for conventional use. It is however poorly suited for scripting and devops workflows:

1. Always an extra init step, and an extra file to manage.
2. The file is difficult to edit via scripts. This requires additional tooling to edit - basic grep/sed isn't good enough as you need a section parser (e.g. there are multiple endpoint = xxx lines under different sections.

I propose shifting to a fully CLI/clap based approach. We could combine this with toml files by having cli args take precedence but in my experience this becomes a pain to maintain. The "optimal" naming scheme is also often different between the two so things drift.

There are situations where having file based configuration is great. One example is updating configs across multiple instances to identical settings - ideally you just copy the file around. Or users that don't want to specify a long list of non-default cli args (though the latter could be done via alias). For this usage I propose environment args. clap supports these out of the box, and does the correct thing. File configuration then becomes a .env file.

An additional bonus is that many deployment tools support env var based flows, e.g. systemd service files let you specify env variables, or even .env files. Similarly in docker.

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This is a rather large breaking change; so I'd like to get consensus on this before proceeding.