Support testing buildpacks compiled in release mode

[edmorley]

Previously libcnb-test only supported compiling the buildpack under test using Cargo's dev profile.

Now, TestConfig::cargo_profile can be used along with CargoProfile to instruct libcnb-package to compile the buildpack using Cargo's release mode (equivalent to --release).

The TestConfig default profile remains CargoProfile::Dev as before.

This:

• Allows buildpack maintainers to opt into release mode, if their buildpack's tests happen to run particularly slowly in debug mode. Whilst release builds take longer, in some cases this is vastly offset by the faster runtime performance of release builds (particularly when the compilation cache is warm).
• (As an added bonus) Could help in the future should we encounter any "release mode only" compiler related bugs, where being able to easily run integration tests in release mode would be invaluable.

An integration test has not been added, since:

• There is no easy way to determine in the test whether the buildpack was actually built in release mode or not (we don't expose the cargo build logs).
• The integration test would be the only one in this repo to run in release mode, so would add to the end to end time due to it having a cold compilation cache, and also bloat the CI cache (due to there being a mix of debug+release builds; something that people using release mode long-term in their own buildpack are unlikely to do).
• Most of this change has coverage via the type system + TestConfig::cargo_profile rustdoc example.
• The release mode is not new, and is already exercised via libcnb-cargo.

Fixes #386.
GUS-W-10912497.