Support custom rustls crypto provider in TlsAcceptor / TlsConnector and ServerTlsConfig / ClientTlsConfig

[janrueth]

Feature Request

Crates

tonic crate in the transport module

Motivation

rustls 0.22.0 introduces custom crypto providers (see #1509).
This allows plugging in different crypto providers.

While there are examples already on how to use rustls circumventing the built-in tls feature, and they can be easily extended to utilize a custom crypto provider, it would be very convenient if one were able to simply pass the provider to the Server/ClientTlsConfig structs.

Supporting custom providers enables a simple path to using tonic in environments where for example FIPS certification of the crypto libraries is required by plugging in a rustls fips-compliant crypto provider.

Proposal

Here is an example for a client to utilize a custom provider: https://github.com/rustls/rustls/blob/main/provider-example/examples/client.rs

In essence, the TlsConnector and TlsAcceptor today call: ClientConfig::builder() and ServerConfig::builder(), see:

tonic/tonic/src/transport/service/tls.rs

Line 35 in 522a8d7

let builder = ClientConfig::builder().with_safe_defaults();

tonic/tonic/src/transport/service/tls.rs

Line 110 in 522a8d7

let builder = ServerConfig::builder().with_safe_defaults();

I propose moving the current code in the new methods to a with_opt_provider constructor, changing these methods to accepting an optional reference to a crypto provider and when this is given calling ::builder_with_provider() with the provided provider, thus enabling the provider.
The new constructors than continues to provide the same API and can simply call with_opt_provider, providing None for the optional provider.

Additionally, enabling to specify the provider in ServerTlsConfig / ClientTlsConfig (a rustls_provider() method) would then allow to expose this to the user as part of the tls feature by calling with_opt_provider in tls_acceptor / tls_connector

Alternatives

Directly accept the optional provider on the new methods. Drawback, API changes.

[LucioFranco]

Related conversation #1615 closing since these are both duplicates.