Redirect Spring cli distribution bundle

This commit adds the '/spring' endpoint that is used to download the
Spring CLI distribution bundle. Instead of relying on the presence of
a local 'spring.zip' file uploaded as part of the application, a
redirect to a configurable repository is used.

It is possible to download both the zip and the tar.gz distribution
by specifying the extension in the url (i.e. /spring.tar.gz provides
the tar.gz distribution.

Fixes gh-31