Add response.content_range() API

[nyurik]

When working with content ranges, all clients would need to perform the same content range parsing based on RFC. Would it be ok to add support for such parsing to the reqwest lib? I could try to provide a PR for that if there is interest.

Update I implemented this as a separate crate - not sure if the reqwest maintainers want to use it directly.

/// HTTP Content-Range response header representation.
pub enum ContentRange {
    /// Regular bytes range response with status 206
    Bytes(ContentRangeBytes),
    /// Regular bytes range response with status 206
    UnboundBytes(ContentRangeUnbound),
    /// Server response with status 416
    Unsatisfied(ContentRangeUnsatisfied),
    /// Header cannot be parsed. This includes non-standard response with status 206
    Unknown,
}

pub struct ContentRangeBytes {
    pub first_byte: u64,
    pub last_byte: u64,
    pub complete_length: u64,
}

pub struct ContentRangeUnbound {
    pub first_byte: u64,
    pub last_byte: u64,
}

pub struct ContentRangeUnsatisfied {
    pub complete_length: u64,
}