Feature request: Support AWSS3.S3Paths

[ericphanson]

Since tensorboard supports logdirs being in s3 directly, it would be nice to be able to write to s3 directly too!

[PhilipVinc]

I don’t have time to work on this and soon will leave for holidays.
However if you open a PR I’ll gladly review it quickly.

this should be as easy as changing the type restriction on the path from a string to a more general path

[ericphanson]

One hurdle is that open on an S3Path returns an FilePathsBase.FileBuffer <: IO object but not an IOStream. So those types would need to be widened too. We could widen to IO if type stability is not a big problem here, or else add a depedency on FilePathsBase and type them as Union{IOStream, FileBuffer}.

Also, there isn't really a common supertype of String and S3Path, but could do Union{String, AbstractPath} if we added a dep on FilePathsBase. Or just Any if type stability here isn't a bottleneck. I'm guessing often the 20 microseconds of dynamic dispatch is less than the cost of IO but I don't really know how TensorBoardLogger works.