Update gcp-resources module to follow style guide

[Copilot]

This PR updates the gcp-resources module to comply with the OpenTelemetry Java style guide requirements while making minimal changes to the codebase.

Changes Made

GCPResourceProvider.java

• Added final keyword to the public non-internal class as required by the style guide
• Improved class organization with proper spacing between static and instance fields

IncubatingAttributes.java

• Moved private constructors to the end of utility classes following the style guide convention
• Applied the same pattern to the nested CloudPlatformIncubatingValues class for consistency

What Was Not Changed

Per the issue requirements:

• @Nullable annotation usage was ignored as instructed
• No new comments were added
• No @AutoService annotation was added since the service is already manually registered via META-INF/services

All tests continue to pass and functionality remains unchanged. The changes are surgical and focused solely on style guide compliance.

Fixes #381.

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