Relax client.credentials-provider requirements

[morozov]

As a workaround for the issue described in #10614, I tried to build a custom AWS credentials provider. I couldn't find the user-facing documentation on how do do that, so I resorted to reading the source code.

My understanding is that the class referenced by the client.credentials-provider property has to return an AwsCredentialsProvider instance from its create() method and also implement the AwsCredentialsProvider interface itself. The former makes sense, since this class effectively acts as a factory, the latter doesn't because the class in question is not a credentials provider, it's a credentials provider factory.

Even the DummyValidProvider, in order to satisfy the requirements, effectively reimplements the StaticCredentialsProvider class from the AWS SDK.

With this constraint removed, instead of having to implement both the AwsCredentialsProvider methods and the static create() method, the provider/factory could implement only the latter:

Before:

iceberg/aws/src/test/java/org/apache/iceberg/aws/TestAwsClientFactories.java

Lines 243 to 253 in 7071dc1

	private static class DummyValidProvider implements AwsCredentialsProvider {
	public static DummyValidProvider create() {
	return new DummyValidProvider();
	}
	@Override
	public AwsCredentials resolveCredentials() {
	return AwsBasicCredentials.create("test-accessKeyId", "test-secretAccessKey");
	}
	}

After:

iceberg/aws/src/test/java/org/apache/iceberg/aws/TestAwsClientFactories.java

Lines 263 to 269 in 2cf0b36

	private static class AwsCredentialsProviderFactory {
	public static AwsCredentialsProvider create() {
	return StaticCredentialsProvider.create(
	AwsBasicCredentials.create("test-accessKeyId", "test-secretAccessKey")
	);
	}
	}

Note that even if/when the original issue is addressed, it would still make sense to relax these constraints to improve the developer experience.

[nastra]

@jackye1995 could you (or anyone on your team) take a look at this one please?