setup-alpine: Allow answer file over HTTP(S)

Current status:

setup-alpine -f currently directly sources the provided answer file. This requires that the answer file is already present on the local file system.

Proposed feature:

To provision a new host quickly, it would be useful if it automatically downloaded remote answer files over HTTP(S). FTP would also be an option, but I'm not sure if it's still in use for tasks like this.

Possible implementation:

If the answer file path starts with a protocol, wget could be used to download the answer file into a temporary file as generated by mktemp. The protocols http://, https:// and ftp:// could be allowlisted to ensure that it will work with wget.