Add support for SASL extensions

[Louis-Tarvin]

This PR builds off of the work done in #442 and #457, adding the missing support for SASL extensions. This allows extensions to be returned from ClientContext::generate_oauth_token as key-value pairs, which then get passed to rd_kafka_oauthbearer_set_token after being converted into the correct format.

Example:

struct RefreshContext;

impl ClientContext for RefreshContext {
    const ENABLE_REFRESH_OAUTH_TOKEN: bool = true;

    fn generate_oauth_token(
        &self,
        oauthbearer_config: Option<&str>,
    ) -> Result<OAuthToken, Box<dyn std::error::Error>> {
        ...
        return Ok(OAuthToken {
            ...
            extensions: vec![
                ("logicalCluster".to_string(), "abc123".to_string()),
            ]

    }
}

I've tested this using the logicalCluster and identityPoolId extensions with Confluent Cloud.