Add an optional argument to HttpServerResponse.sendFile() to control whether the response is ended

Add an optional argument to `HttpServerResponse.sendFile()` that indicates whether the response should be ended.

Currently, `sendFile` always ends the response. This prevents sending data from more than one file.

This would be similar in nature to `Pipe.endOnComplete()`.

This option would be also be available on the `sendFile(filename, offset)` and `sendFile(filename, offset, length)` variants as well, so partial files could also be sent.

Use case: We have an S3 storage service where multipart objects are stored in multiple files. We can send single part S3 objects with `sendFile`. But, when sending multipart objects (ranges), we must compose `Pipe` futures with `endOnComplete(false)`, which does not take advantage of the zero-copy IO features of Linux.

I would not be able to contribute, but would be able to test the feature.

Thanks!