cli: allow saving collector credential to file; update help text

[inahga]

Alternative approach to divviup/janus#2221, let the user provide --save to save the correctly encoded credentials to file, and make it more explicit as to what portions of the CLI output need to be saved.

Also update the help text to use the collector credential nomenclature.

[inahga]

@jbr any thoughts on this PR?

[jbr]

This seems like an unconventional way to construct a path in rust (using +). I'm also slightly concerned that we haven't guarded against name including dangerous path characters like slashes, dots, and tildes. On one hand, people are unlikely to script this particular part of divviup-api with untrusted inputs. On the other, we probably should do something reasonable regardless of the --name.

Suggested change

	let mut file = File::create(name + ".json")?;
	let path = std::path::Path::new(name).with_extension("json");
	let mut file = File::create(path)?;

or maybe

Suggested change

	let mut file = File::create(name + ".json")?;
	let path = std::env::current_dir()?.with_file_name(name).with_extension("json");
	let mut file = File::create(path)?;

or with the same behavior but slightly more conventional appearance:

Suggested change

	let mut file = File::create(name + ".json")?;
	let mut file = File::create(format!("{name}.json"))?;

[jbr]

we may also want something like

println!("Saved collector credential to {path}. Keep this somewhere safe!");

[inahga]

I liked the current_dir()... approach, since that offloads all the platform-specific gotchas onto std.

[jbr]

🚀