Add support for composite buildpack targets during publication

[joshwlewis]

This PR reads [[metadata.targets]] from a composite buildpack's buildpack.toml to determine what targets the composite buildpack is intended to be compiled for.

Why [[metadata.targets]]?

We're not actually sure if [[targets]] is legal as a part of a composite buildpack descriptor, so libcnb.rs has not modeled it as noted here:

https://github.com/heroku/libcnb.rs/blob/a4e48fde0b502eec1ce1d0b811bb1ac61beed101/libcnb-data/src/buildpack/mod.rs#L182-L185

The multi-platform rfc seems to indicate that this will eventually be part of package.toml:

https://github.com/buildpacks/rfcs/blob/abd3ba28d7648e03a9c2074a88dae9fe3b069683/text/0000-multiarch-builders-and-package.md?plain=1#L754-L776

But package.toml is not a part of the spec, and this field isn't implemented in libcnb.rs or pack yet. Using it now would break cargo libcnb package workflows. Setting [[targets]] in package.toml also seems like something that is an output of cargo libcnb package, rather than an input to it.

I don't think metadata.targets will be permanent, but rather a stopgap until multi-arch support lands in pack and libcnb.rs at a future date.