Add cxx interop build pipeline

[gjcairo]

This PR adds a new target to docker-compose that builds a new project that depends on this one, with C++ interoperability enabled, with the goal of adding a new CI pipeline that checks the build succeeds.

Motivation:

To make sure that C++ interoperability can be used on projects that depend on swift-nio-ssl.

Modifications:

• Added a new target to docker-compose that creates a new package that depends on swift-nio-ssl, and builds it with C++ interoperability enabled.
• Will request a new CI pipeline that runs this new target.

Result:

We will ensure this projects builds properly with C++ interoperability enabled.

[Lukasa]

I'm not confident this is enough: we should also add imports of all the targets we're depending on into the test product. That should force us to actually build everything and confirm they work.

[Lukasa]

One quick nit and then we're good.

[yim-lee]

@swift-server-bot add to allowlist

[gjcairo]

@yim-lee can thanks for dealing with the pipelines!