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 Speed up user initialization #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.

[Michael-Hollister]

When copying the example for testing the branch, I got this error below. Perhaps there should be additional cargo features that only enable default-tls/rustls-tls without including cookies?

error: failed to parse manifest at `Cargo.toml`

Caused by:
  feature `reqwest/default-tls` in dependency `goose` is not allowed to contain slashes
  If you want to enable features of a transitive dependency, the direct dependency needs to re-export those features from the `[features]` table.

[LionsAd]

I personally find the many cfg things to add a lot of complexity.

I am okay with some very restricted changes, but this PR feels overkill to me.

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.

[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.