allow plus sign in fronts

on r2 we could have fronts with + in the name (tag combiners)  however on ngw the plus is stripped out, so we need to add the plus as a possibility.

if we do create a front with a plus in the name, the pressed file is in a directory on s3 with a plus in the name.  if we want to gET it with http we need to escape the plus as a %2b otherwise it won't find it.  We can't just do a general escaping of all characters invalid in a path segment as forward slashes are actually used in S3 with their semantic meaning not just as a character, so just escaping the plus alone for now.