Add C++ proxy classes for UserIdProvider.java and BorgUserIdProvider.java classes.
Add C++ proxy classes for UserIdProvider.java and BorgUserIdProvider.java classes.

These proxy classes are not used by the agent yet.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170285296

GoogleCloudPlatform/cloud-debug-java,GoogleCloudPlatform/cloud-debug-java,GoogleCloudPlatform/cloud-debug-java,GoogleCloudPlatform/cloud-debug-java