blob/s3: Add content length parameter in signing URL about go-cloud

[vangent]

I explored this a bit:

• It looks like AWS allows you to set ContentLength in PutObjectInput as you say. I haven't tested it. Does it require an exact match, or is a maximum length?
• GCS allows a range, via storage.GenerateSignedPostPolicyV4: https://stackoverflow.com/q/63230403.
• I don't see any way to do this for Azure.
• I think fileblob could support an exact match.

This will be hard for you to contribute, because in order to test it we'd want to add a driver test, which will update all of our golden files and requires access to our test projects, but if we can find reasonable semantics that are supported across the providers I can add it.

[mrth1995]

Hi vangent thanks for the response!

As per my exploration on AWS, its exact content length.
So I guess only Azure that doesn't support this? Are you okay if we add this feature to go-cloud?
It'll be helpful for given usecase I think.

Thanks!

[vangent]

I looked into this some more. GCS doesn't support this either, at least not in the same way; it has https://pkg.go.dev/cloud.google.com/go/storage#GenerateSignedPostPolicyV4, but that's for POST, not PUT.

Given the lack of consistency across providers, I don't think it makes sense to add to Go CDK.

You can use As (https://gocloud.dev/concepts/as/) and SignedURLOptions.BeforeSign (https://pkg.go.dev/gocloud.dev/blob#SignedURLOptions) to set the PutObjectInput.ContentLength for S3. Here's an example using As for writes that should be similar: https://pkg.go.dev/gocloud.dev/blob#example-WriterOptions.