add client certificate and grpc backend builder options

[dgryski]

No description provided.

[joeshaw]

Should other SSL-related things (CertHostname, CACert, Ciphers, SNIHostname, SSLMaxVersion, SSLMinVersion) also imply UseSSL(true)? Feels like either all of them should or none of them should, and we should make it clear in the docs if so.

[dgryski]

This behaviour was copied from the Rust SDK.

    /// Provide the given client certificate to the server as part of the SSL handshake.
    //
    /// Setting this will enable SSL for the connection as a side effect. Both
    /// the certificate and the key to use should be in standard PEM format;
    /// providing the information in another format will lead to an error. We
    /// suggest that (at least the) key should be held in something like the
    /// Fastly secret store for security, with the handle passed to this function
    /// without unpacking it via [`Secret::plaintext`]; the certificate can
    /// be held in a less secure medium.
    ///
    /// (If it is absolutely necessary to get the key from another source, we
    /// suggest the use of [`Secret::from_bytes`]).
    pub fn provide_client_certificate(
        mut self,
        pem_certificate: impl ToString,
        pem_key: Secret,
    ) -> Self {
        self.use_ssl = true;
        self.client_certificate_info = Some((pem_certificate.to_string(), pem_key));
        self
    }

The other SSL-related calls also set use_ssl = true in the Rust SDK. I'll update our implementation as well.

[cee-dub]

This LGTM.

[joeshaw]

Doc suggestions below. Otherwise LGTM.