Support building and linking C/C++ code to Rust `oak_loader`

[tiziano88]

We will need to keep supporting implementing some of the runtime in C++, and therefore:

• call from Rust to C++ (e.g. implement some part of a pseudo-node in C++ that can be invoked from Rust)
• call from C++ to Rust (e.g. the C++ implementation of a pseudo-node may need to read and write to channels)

In order to build the C++ code, we will probably keep relying on Bazel, and invoke the relevant command from a Rust build.rs script.

Ref.:

• 723

• https://doc.rust-lang.org/cargo/reference/build-scripts.html#cargorustc-link-libkindname
• https://doc.rust-lang.org/rustc/command-line-arguments.html#option-l-link-lib

[tiziano88]

@daviddrysdale assigning to you to figure out what is actually necessary, and how to go about it. Perhaps it would be useful to target a specific piece of functionality that we expect to be keeping in C++, e.g. the roughtime client.

cc @conradgrobler

[tiziano88]

In fact I think roughtime is particularly nice in terms of doing this incrementally, since it may be encapsulated as a stateless C++ function that gets invoked from Rust when necessary to retrieve the time (if we are fine with re-establishing the network connection every time, as opposed to keeping it around as part of a local state).

[daviddrysdale]

No longer relevant after #1016 – we now have a pure Cargo build for the server-side code.