Move configuration into config file

[Leonidas-from-XIV]

Currently the configuration is hardcoded into bin/config.ml. This is workable but poses a bunch of issues:

• It needs to be rebuilt if the config changes
• It's not possible to ship more than one config (stable vs test)
• The changing of the config is currently done in the workflows via sed, this is duplicated code and fairly fragile if anything in config.ml changes
• Currently the metadata is configured via CLI, if the config file could be configured via CLI, the metafile(s) could be read from the config instead

Thus it would be better to use some kind of configuration format. The format itself does not matter that much.

• JSON is not a great format because it is fairly verbose and doesn't allow comments. But we already use Yojson
• INI is fine, if we can find a decent implementation
• TOML is fine too

In general whoever ends up picking this up has a decent degree of freedom which solution to choose. The config files are fairly simple.