Add feature to allow enabling FIPS in rustls

[behos]

In order to be able to enable fips when depending on the zookeeper client we need to allow setting the rustls feature.

[kezhuw]

This does not sound enough to be fips compatible ?

https://docs.rs/rustls/latest/rustls/manual/_06_fips/index.html

Seems that we have to config an option to enable fips compatible crypto provider ?

[kezhuw]

Seems that one could enable this implicit outside zookeeper-client by:

rustls::crypto::default_fips_provider().install_default()
    .expect("default provider already set elsewhere");

But we do have option to do this explict regardless of dependency or environment.

https://docs.rs/rustls/latest/rustls/crypto/fn.default_fips_provider.html