Implement bom.json caching on rebuilds with no node module changes

[sophiewigmore]

Per discussion in the initial implementation of the Node Module BOM buildpack, it would be good if we implemented caching for the bill of materials for the node modules if the node_modules directory does not change. This will save up to 3 seconds on rebuilds where the modules do not change.

Natively, we can't cache the BOM because it requires changes upstream. However, this can be achieved by storing the generated bom.json in a cache layer, and reusing it for the BOM if the node_modules directory has not changed.

Acceptance

The outcome of this issue should be a release of the Node Module BOM buildpack such that when I perform a rebuild of an NPM app (with that buildpack release included in the build) with no node module changes, the rebuild uses the cached BOM, and does not perform BOM tool installation and BOM generation.