Configure Relay to accept self-signed certificates

[jjbayer]

Use case: A user runs Relay and wants to connect to sentry.io through a proxy.

Currently certificates are compiled in because we use reqwest with the native-tls-vendored feature. But we could add a config option that points to a self-signed "upstream certificate", and load that via https://docs.rs/reqwest/latest/reqwest/struct.ClientBuilder.html#method.add_root_certificate.

[linear]

RELAY-184

[Dav1dde]

Maybe we just need to init openssl via https://docs.rs/openssl-probe/latest/openssl_probe/fn.try_init_openssl_env_vars.html

Also I am not sure if it makes sense to vendor openssl, we already have an openssl dependency through Kafka. There is a possibility by vendoring we break the discovery of certificates from the host environment.

I would largely prefer if we can keep all things certificates to the host environment.

[jjbayer]

I would largely prefer if we can keep all things certificates to the host environment.

Agreed! I assumed there might be a reason for the vendoring, but if we can replace it, even better.