send_file: allow explicit etag passing

[ThiefMaster]

send_file should have a kwarg for explicitly specifying an etag so it can be used even without a physical file where it would be autogenerated.

Right now one has to copy&paste code (the stuff from if conditional:) into their if they want to include an etag for a file that does not represent a physical file on disk, because make_conditional should ideally be used on a response that already has an etag set...

This depends on #1850 since it moves send_file from Flask to werkzeug.

[davidism]

Would allowing add_etags to be a string or list of strings in addition to True be a good idea?

[davidism]

@ThiefMaster let me know if that's what you had in mind, or something else.

[ThiefMaster]

Not sure if mixing bool and string is that clean but it certainly sounds like a solution that would work.

[kx-chen]

Will be working on this.

[davidism]

I couldn't find any reference for allowing multiple ETag header values in the response, so I'm only implementing str, not list. Taking advantage of the fact that we're renaming other parameters during the move, I renamed the parameter to etag, since the previous name seemed incorrect on a few levels. I added a corresponding rename and warning to Flask (to go with the other two so far), although since this defaults to True it's unlikely to come up much.