Add layer "pull" support via generic code archive download

[dominikschubert]

Generalizes our "fetch archive content via URL & mount it somewhere" feature.

We can now use LOCALSTACK_CODE_ARCHIVES with a json value of the following format:

[
  {
    "url": "https://example.com/mycodearchive.zip",
    "target_path": "/var/somewhere"
  },
 // ... more fetch & mounting entries
]

For each entry it will fetch the file at the url (expected to be a zip package) and extract it to the target_path.

[dfangl]

LGTM, at least the parts I didn't write :)