feat: Add ConnectionID for BigQuery Object Tables secure external access configuration

[vasilyl]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: N/A
• Related: feat: Add BigQuery Agent Analytics Plugin #738

2. Or, if no issue exists, describe the change:

Problem:
When offloading large text or media payloads to Google Cloud Storage (GCS), analytical queries on the logged telemetry database need a secure, unified way to link the external GCS references back to BigQuery. BigQuery Object Tables allow querying external GCS objects securely, but require an explicit BigQuery connection ID.

Solution:
Introduce a new configuration parameter ConnectionID string to the plugin Config struct (in config.go), and initialize it to an empty string by default in DefaultConfig(). This provides a clean, standard location to pass the BigQuery external connection resource path, preparing the analytics plugin to generate secure metadata references (authorizer field in external object refs).

Testing Plan

Verified that the ConnectionID field is successfully defined on the Config struct, and that it defaults to an empty string when calling DefaultConfig() in testing.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.