refactor(update-agent-loader): pass configuration in explicitly

[TheButlah]

Following changes were made:

• add orb- prefix, as conventional in our workspace
• set version to 0.0.0. as conventional in our workspace
• Added a Config struct which has a from_env function, call this in main, and pass this into the rest of the code to be explicit about configuration of the program (and make it more possible to test the code)
• Removed build.rs as it both doesn't make sense to check target os and error (the code will already error if its platform specific) and was non-hermetic (randomly generates build-time compiled constants from a non-seeded RNG)
• As per @alekseifedotov's request, I kept the constraint that if a public key is not provided at build time, a randomly generated private key will be used. the difference now is that this random generation happens at run time, not build time. And with the dependency injection, it is now possible to see that random pubkey in tests.