feat: Add AWS credential parameters to create_client()

- Add aws_access_key_id, aws_secret_access_key, aws_session_token, and region_name parameters
- Pass credentials through to S3StorageAdapter and boto3.client()
- Enables multi-tenant scenarios with different AWS accounts
- Maintains backward compatibility (uses boto3 default credential chain when omitted)
- Add comprehensive tests for credential handling
- Add examples/credentials_example.py with usage examples

Fixes credential conflicts when multiple SDK instances need different credentials.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

examples/credentials_example.py

@@ -0,0 +1,101 @@
+"""Example: Using explicit AWS credentials with DeltaGlider.
+
+This example demonstrates how to pass AWS credentials directly to
+DeltaGlider's create_client() function, which is useful when:
+
+1. You need to use different credentials than your environment default
+2. You're working with temporary credentials (session tokens)
+3. You want to avoid relying on environment variables
+4. You're implementing multi-tenant systems with different AWS accounts
+"""
+
+from deltaglider import create_client
+
+
+def example_basic_credentials():
+    """Use basic AWS credentials (access key + secret key)."""
+    client = create_client(
+        aws_access_key_id="AKIAIOSFODNN7EXAMPLE",
+        aws_secret_access_key="wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY",
+        region_name="us-west-2",
+    )
+
+    # Now use the client normally
+    # client.put_object(Bucket="my-bucket", Key="file.zip", Body=b"data")
+    print("✓ Created client with explicit credentials")
+
+
+def example_temporary_credentials():
+    """Use temporary AWS credentials (with session token)."""
+    client = create_client(
+        aws_access_key_id="ASIAIOSFODNN7EXAMPLE",
+        aws_secret_access_key="wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY",
+        aws_session_token="FwoGZXIvYXdzEBEaDH...",  # From STS
+        region_name="us-east-1",
+    )
+
+    print("✓ Created client with temporary credentials")
+
+
+def example_environment_credentials():
+    """Use default credential chain (environment variables, IAM role, etc.)."""
+    # When credentials are omitted, DeltaGlider uses boto3's default credential chain:
+    # 1. Environment variables (AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY)
+    # 2. AWS credentials file (~/.aws/credentials)
+    # 3. IAM role (for EC2 instances)
+    client = create_client()
+
+    print("✓ Created client with default credential chain")
+
+
+def example_minio_credentials():
+    """Use credentials for MinIO or other S3-compatible services."""
+    client = create_client(
+        endpoint_url="http://localhost:9000",
+        aws_access_key_id="minioadmin",
+        aws_secret_access_key="minioadmin",
+    )
+
+    print("✓ Created client for MinIO with custom credentials")
+
+
+def example_multi_tenant():
+    """Example: Different credentials for different tenants."""
+
+    # Tenant A uses one AWS account
+    tenant_a_client = create_client(
+        aws_access_key_id="TENANT_A_KEY",
+        aws_secret_access_key="TENANT_A_SECRET",
+        region_name="us-west-2",
+    )
+
+    # Tenant B uses a different AWS account
+    tenant_b_client = create_client(
+        aws_access_key_id="TENANT_B_KEY",
+        aws_secret_access_key="TENANT_B_SECRET",
+        region_name="eu-west-1",
+    )
+
+    print("✓ Created separate clients for multi-tenant scenario")
+
+
+if __name__ == "__main__":
+    print("DeltaGlider Credentials Examples\n" + "=" * 40)
+
+    print("\n1. Basic credentials:")
+    example_basic_credentials()
+
+    print("\n2. Temporary credentials:")
+    example_temporary_credentials()
+
+    print("\n3. Environment credentials:")
+    example_environment_credentials()
+
+    print("\n4. MinIO credentials:")
+    example_minio_credentials()
+
+    print("\n5. Multi-tenant scenario:")
+    example_multi_tenant()
+
+    print("\n" + "=" * 40)
+    print("All examples completed successfully!")