remote - stage files from remote urls

Usage:

# Specify the remote source kind
kind: remote

# Optionally specify a relative staging filename.
# If not specified, the basename of the url will be used.
# filename: customfilename

# Optionally specify whether the downloaded file should be
# marked executable.
# executable: true

# Specify the url. Using an alias defined in your project
# configuration is encouraged. 'bst source track' will update the
# sha256sum in 'ref' to the downloaded file's sha256sum.
url: upstream:foo

# Specify the ref. It's a sha256sum of the file you download.
ref: 6c9f6f68a131ec6381da82f2bff978083ed7f4f7991d931bfa767b7965ebc94b

See built-in functionality doumentation for details on common configuration options for sources.