[github] Add `HttpCache::in_dir` constructor to allow for custom cache placement

[ginger51011]

Before only the home dir could be used, this makes it a bit more ergonomic if the user has other desires.

[augustuswm]

Makes sense to me. Only suggestion is that in_home_dir get replaced with:

pub fn in_home_dir() -> BoxedHttpCache {
    Self::in_dir(&dirs::home_dir().expect("Expected a home dir"))
}

[ginger51011]

Makes sense to me. Only suggestion is that in_home_dir get replaced with:

Good point, added that!