new feature: allow GCP unauthenticated requests

[jdockerty]

Feature Description

Allow unauthenticated requests to a GCS (or emulated) bucket.

Problem and Solution

This enables simpler testing and looks to be the same reasoning for why the Java implementation has this too.

Additional Context

I ran into this whilst looking at adding GCS support to iceberg-rust.

Happy to help contribute this with some guidance 👍

Are you willing to contribute to the development of this feature?

• Yes, I am willing to contribute to the development of this feature.

[Xuanwo]

Thanks a lot in advance!

To support unauthenticated requests, we take a reference from S3::allow_anonymous.

We might need to have the following fields for gcs config:

• allow_anonymous
• disable_disable_vm_metadata
• disable_config_load

And implement something like this for gcs:

opendal/core/src/services/s3/core.rs

Lines 111 to 144 in b37191d

	async fn load_credential(&self) -> Result<Option<AwsCredential>> {
	let cred = self
	.loader
	.load_credential(self.client.client())
	.await
	.map_err(new_request_credential_error)?;
	if let Some(cred) = cred {
	// Update credential_loaded to true if we have load credential successfully.
	self.credential_loaded
	.store(true, atomic::Ordering::Relaxed);
	return Ok(Some(cred));
	}
	// If we have load credential before but failed to load this time, we should
	// return error instead.
	if self.credential_loaded.load(atomic::Ordering::Relaxed) {
	return Err(Error::new(
	ErrorKind::PermissionDenied,
	"credential was previously loaded successfully but has failed this time",
	)
	.set_temporary());
	}
	// Credential is empty and users allow anonymous access, we will not sign the request.
	if self.allow_anonymous {
	return Ok(None);
	}
	Err(Error::new(
	ErrorKind::PermissionDenied,
	"no valid credential found and anonymous access is not allowed",
	))
	}

Please let me know if you have any other questions.