Support unmanaged layers

[jeffsw]

Need to ensure support for layers we don't build & update.

If a layer contains C libraries, etc. lambda-zip currently doesn't have a way to build them. More complex is that Python's tools don't necessarily work on MacOS where users may want to use it. For example, below command is needed to produce a psycopg2 layer using a pip package which vendors the required pgsql library. This doesn't work on MacOS and Mac users are likely to need docker to build the layer.

pip install \
  --no-deps \
  --platform manylinux2014_x86_64 \
  --python-version 3.9 \
  --only-binary=:psycopg2: \
  --target=${HOME}/tmp/psycopg2_bin_layer/python \
  psycopg2-binary