make it possible to disable cookies and optimize startup

[jeremyandrews]

• add new cookies feature, enabled by default
• optimize GooseUser startup if cookies are disabled
• only compile Reqwest Cookies if cookies feature is enabled in Goose
• document
• fixes https://github.com/tag1consulting/goose/pull/557

By making this a compile-time feature, we can enforce at compile-time that nobody expects cookies to work properly when using this startup optimization. Attempts to use Reqwest's cookie-related functions will fail if the Goose cookie feature is disabled. This also removes the cookie dependencies when it's not needed.

[jeremyandrews]

As far as I understand the slowdown is mostly from reqwest cookie feature itself, so removing that is fine, but cluttering the code with cfg feels like premature optimization.

Yes, the added config option is not an attempt to speed up or optimize anything. It's so the compiler can catch if someone 1) disables cookies, and then 2) tries to use cookie functinality: it will fail to compile. Without the config option, a person could disable cookies but use the code causing a panic or other undefined errors.