Allows user or another buildpack to set `HEROKU_USE_SYSTEM_RUBY`. This will:

- Skip our logic for installing ruby
- Trigger a `which ruby` command to run, if it returns non-zero exit the build will not proceed
- Trigger a `ruby -v` command to run, if it returns non-zero exit the build will not proceed
- Includes a warning message stating that turning this on effectively disables any guarantees of support

The main driver is allowing for experimentation of replacing a Ruby binary.

## Allow `*` stacks

As stacks are going away, we can begin to remove the hard requirement that this buildpack only be run with known stacks. To clarify support I added stack detection warning messages.


## Refactor Ruby and Bundler version logic

I've learned a lot since I wrote this original code. I like the way Ed modeled https://github.com/heroku/buildpacks-python/blob/main/src/python_version.rs. 

This refactor allows us to represent the difference between a build where we are responsible for installing Ruby and one where we're not. We need to be able to bust layer caches based on these versions no matter what so I introduced a `RubyCacheKey` which is not guaranteed to also be an installable `RubyVersion`.

The logic in `commons/gemfile_lock.rs` was not great, and it contained structs that seemed like they would better live somewhere else.

Functions in `steps/` for bundler and ruby versions (download and install) were very small logically work better if grouped with their associated version logic.

## Updated section variables in main.rs

Update variables used for logging runtime duration to match the thing they're measuring. This reads better and helps understand what header is being closed by `done()` calls.