Store PNGs to google cloud

[minnerbe]

This PR extends the RenderTilesClient to be able to store rendered tiles¹ to cloud storage².

¹ Only PNGs are tested so far. The actual writing is done using ImageIO.write to write to a ByteArrayOutputStream, which is subsequently uploaded. Every format that is supported by that workflow should work, but the parameters --convertToGray and --quality are ignored right now.

² Only GCS is supported so far. For AWS, the changes to GCS shouldn't be too dramatic (since the actual object access is implemented using N5's KeyValueAccess), but I haven't spent the time figuring out authentication details.

Further caveats:

• We currently use an old version of N5GoogleCloud (4.1.1), which has bugs in URI handling. The bugs should be fixed in version 5.X.X, but for now all URI handling is done using other tools.
• The KeyValueAccess interface does not allow for fine-grained control over the upload. This results in all images being tagged with type application/octet-stream. To preview the images directly in GCS, one has to manually set the type to image/png.

I've tested that

• File system paths are the same as before the refactoring.
• Writing both images and masks works.
• GCS paths are correct.
• The paths are correctly stored in the hack stack and work (see here).

Let me know if that's what you intended, @trautmane!

[trautmane]

Looks great! I'll use this branch to add spark pipeline stuff that wraps what you've done here.