Support packaging additional buildpack artifacts/assets

[joshwlewis]

In my buildpack directory, I have a inventory.toml file. It includes a list of binaries, their version, and their sha. Ideally, I read this file during a build to help figure out which binary to download.

When I run cargo libcnb package the resulting buildpack does not include inventory.toml. So I cannot read this file during the build.

I believe my options are limited to using include_str! to read the file during compilation, merging this list into my buildpack.toml metadata, or scripting a cp into the packaging process.

It would be nice if libcnb had an API for automatically including files like this in the buildpack output.