Start adding S3 support for VRTStack

[scottstanie]

Basic support for reading from S3 added, with a focus on the VRTStack. Allows us to do

s3_urls = ['s3://my-bucket/myfile.tif', ...]
r = io.VRTStack(s3_urls, outfile='stack_on_s3.vrt')

The PathOrStr type has been updated to include the GeneralPath protocol which describes the interface for our minimal S3Path.
Note that I considered adding some optional dependency like https://github.com/fsspec/universal_pathlib or https://github.com/liormizr/s3path or ... (there seem to be dozens of attempts). I thought we'd hold off until there's a strong need to to more file maniuplations beyond s3path.exists().

TODO: enable or tune some of these options if we see slow performance for block iteration https://developmentseed.org/titiler/advanced/performance_tuning/