Make Call, Request, Response public

[jcamiel]

In Hurl 3.0.0, the public method run returns a Result<HurlResult, String>.

In the graph structs, there are private structs Call, Request, Response:

pub struct HurlResult {
    pub entries: Vec<EntryResult>,
    pub time_in_ms: u128,
    //...
}

pub struct EntryResult {
    pub entry_index: usize,
    pub calls: Vec<Call>,
    pub captures: Vec<CaptureResult>,
   //...
}

pub struct Call {
    pub request: Request,
    pub response: Response,
    pub timings: Timings,
}

The types Call, Request, Response should be public and exported by the crate hurl.