Allow direct access to modify the TLS config

[n1ght-hunter]

Motivation

For work we need direct access to the tls config to customise the ServerCertVerifier.
Also direct access allow any further config changes to made without any changes to tonic

Solution

To solve access to the tls i have added a method to ClientTlsConfig called modify_config(could be named somthing else maybe).

 pub fn modify_config<F>(self, f: F) -> Self
    where
        F: Fn(&mut tokio_rustls::rustls::ClientConfig) + Send + Sync + 'static,
    {
        let modify_config = ModdifyConfigFn(std::sync::Arc::new(f));
        ClientTlsConfig {
            modify_config: Some(modify_config),
            ..self
        }
    }

this allows users to directly modify tokio_rustls::rustls::ClientConfig to fit their needs.
this function is called in TlsConnector::new after all tonic config changes are set