Feature Request: Support explicit service account credential path

# Problem Statement

When running the Java profiler agent on-premises or in environments where the application already uses `GOOGLE_APPLICATION_CREDENTIALS` for a different Google Cloud service,
there is no way to provide separate credentials for the profiler.

# Current Behavior

The profiler agent currently only supports authentication via:
1. `GOOGLE_APPLICATION_CREDENTIALS` environment variable (calls `grpc::GoogleDefaultCredentials()`)
2. Application Default Credentials from GCP metadata service (not available on-premises)

# Use Case

We have a Java application running on-premises (self-managed Kubernetes cluster) that:
- Uses Google Retail API with credentials at `/config/secrets/google-retail-service-account.json`
- Sets `GOOGLE_APPLICATION_CREDENTIALS=/config/secrets/google-retail-service-account.json`
- Needs to send profiling data to Cloud Profiler using **different credentials**

Currently, we cannot use both services simultaneously because they conflict over the `GOOGLE_APPLICATION_CREDENTIALS` environment variable.

# Proposed Solution

Add a new command-line flag: `-cprof_service_account_json_file` that allows specifying an explicit path to a service account JSON file for profiler authentication.

# Example Usage
```bash
-agentpath:/opt/cprof/profiler_java_agent.so=-cprof_service=myapp,-cprof_service_account_json_file=/etc/gcp/profiler-credentials.json
```

## Related Issues

This affects anyone running the profiler:
- On-premises or in non-GCP environments
- Alongside other Google Cloud services (Retail API, Vertex AI, etc.)
- With security requirements for separate service accounts per service

## Additional Context

We're willing to contribute the implementation if this approach is acceptable to the maintainers.