add force-engine feature for enabling engine module

[neuronull]

In one of our (vector.dev) project's dependencies (rdkafka), a hard dependency on openssl Engine support was added. This resulted in cross compilation errors for our project (since openssl-src was not enabling the engine support), the findings were documented here:

https://github.com/vectordotdev/vector/pull/10302

Allowing this engine feature support to be configurable would help unblock us.

Thanks!

[alexcrichton]

I'd recommend if you can doing an end-to-end integration test with this. This PR, for example, doesn't add a Cargo feature so there's no easy way to actually turn on this cfg other than with RUSTFLAGS which is probably not what you intended.

[neuronull]

Ah yes... I think that is all I was missing. I just completed cross compiling our project to musl, and patching the openssl-sys crate to use this fork and the new feature, and it succeeded.

[neuronull]

Thank you! 🙏