Add CLI support for owner invitation management

[carols10cents]

When someone does cargo owner --add, we're now sending the person an invitation that they can accept or decline before they become an owner. Currently, though, the acceptance part (and seeing that you have an invitation at all) only happens in crates.io's interface.

It'd be nice if cargo owner also had these options:

• --list or --list-invitations or similar, which would list any pending invitations for the current user as specified by the token cargo sends to crates.io
• --accept [cratename] or similar, which would accept a pending invitation for the current user to become an owner of a crate they were invited to
• --decline [cratename] or similar, which would decline a pending invitation for the current user so that they don't become an owner of a crate they were invited to
• These should be documented in the output of cargo owner --help

These should all be able to use the same API routes that the crates.io ember UI uses; there are some tests exercising these routes around here.

[epage]

I would lean towards waiting on #11879 so we can do cargo owner invitations

• cargo owner invitations (show them)
• cargo owner invitations --accept <name>
• cargo owner invitations --decline <name>