Implementation of Server {Login|Logout} commands

[mchf]

Problem

We need authorization token to be able to proceed with subsequent commands

Solution

agama server login asks for JWT and stores it into a well-known file
agama server logout deletes stored JWT

Testing

• Tested manually

[coveralls]

coverage: 74.141% (-0.3%) from 74.401%
when pulling 4754ad9 on agama-cli_login_subcommand
into 987e8f7 on master.

[imobachgs]

I made a few comments (nothing critical), although I would probably only allow reading the password from stdin or, at most, from an environment variable. Of course, as a fallback, I would ask interactively (as you did). I guess it would be more flexible and simpler to implement :-)

[imobachgs]

I am not sure about the help text. Perhaps @mvidner could have a look.

[imobachgs]

No need to be public.

Suggested change

	pub fn jwt() -> anyhow::Result<String> {
	fn jwt() -> anyhow::Result<String> {

[mchf]

i made it public to make it available for possible further use throughout the agama, if you don't think it will ever happen i'll remove it.

[imobachgs]

We can make it public when we need it to be public 😉. If we want to make it reusable, perhaps it should be in agama-lib.

[imobachgs]

It looks good overall (although I would like to have the text reviewed by @mvidner). But that's something we can do when we review the CLI's overall help, which is far from ideal.

As a note, I really think we should read stuff from stdin, but this PR implements what it was in the PBI. So I will create another one for the future.

Last but not least, please, do not "force-push" during a review. It makes it harder to review the changes (for instance, the Compare link from GitHub might be useless then).