Removing client mutability

[BRLunt]

This PR removes &mut self from methods in the OpenAIClientimplementation as suggested in #168.

• it removes the response_headers field from rhe OpenAIClient

This change was made to avoid the need to clone the client when making concurrent or repeated requests improving ergonomics.

Since the response headers were public, this is a breaking change IMO.

#[derive(Debug)]
pub struct OpenAIClient {
    api_endpoint: String,
    api_key: Option<String>,
    organization: Option<String>,
    proxy: Option<String>,
    timeout: Option<u64>,
    headers: Option<HeaderMap>,
    pub response_headers: Option<HeaderMap>,
}

[dongri]

@BRLunt Thank you for creating the PR. The reason I used &mut is that the rate limit information is returned in the headers of the API response, and this was implemented to handle that. However, this PR removes that functionality—what are your thoughts on this?