Add as_url method

[pjbull]

It would be great to be able to generate URLs that can be used.

S3Path("s3://bucket/file.txt").as_url()
# > https://bucket.amazons3.com/file.txt 

S3Path("s3://bucket/file.txt").as_url(presign_expiry_min=100)
# > https://bucket.amazons3.com/file.txt?presign=accesstokenwithexpiry