Centrally Manage Dependency Versions for google-auth-library and the rest of the libraries in google-cloud-java

## The Problem: Auth Dependencies Are Managed Separately
Currently, the `google-auth-library-java` module manages its third-party dependencies (like **Guava**, **Protobuf**, **Jackson**, and **JUnit**) separately within its own local `` block. The rest of the monorepo manages identical dependencies inside `gapic-generator-java-pom-parent`.

This duplicates versions and increases the risk of dependency drift across the monorepo:
* `google-auth-library-java/pom.xml`
* `sdk-platform-java/gapic-generator-java-pom-parent/pom.xml`

Because `com.google.auth` and `com.google.cloud` deploy under different staging profiles on Maven Central, they are uploaded to separate staging repositories. If `google-auth-library-java` references unreleased in-development versions of `com.google.cloud` parent POMs or BOMs, a **circular release staging deadlock** is created at release time.

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## Proposed Solutions

### Option 1: Extract Version Properties to `google-cloud-shared-config`
Move shared version properties (like ``) into the external parent POM `com.google.cloud:google-cloud-shared-config` from which both modules inherit.
* **Pros**:
* Single, definitive source of truth at the parent configuration level.
* Completely clean code structure with zero duplicate config.
* **Cons**:
* **Rigid Release Pipeline**: Upgrading a dependency (like an urgent security CVE patch) requires releasing a new version of `google-cloud-shared-config` to Maven Central first, waiting for Central's CDN to sync, and then updating the parent references in both projects.

### Option 2: Extract Versions to a Common Base BOM Module (in Monorepo)
Introduce a lightweight, internal BOM module (e.g., `base-shared-dependencies`) in this monorepo containing **only** the third-party library versions. Both the auth library and client library BOMs would import it.
* **Pros**:
* Prevents circular staging deadlocks by maintaining a strictly acyclic dependency graph.
* Single source of truth located directly within the same monorepo.
* **Cons**:
* **Release Staging Delay**: Due to staging profile boundaries, releases must be built sequentially (waiting for the base BOM to fully sync to Maven Central before releasing the auth library, and then repeating before client libraries can release).

### Option 3: Keep It as It Is and Use Automation to Update
Keep the properties blocks isolated inside their respective directories, but coordinate updates strictly using **Renovate Bot** or other CI scripting to bump them in parallel.
* **Pros**:
* **Maximum Autonomy**: The auth team can delay upgrades to preserve legacy client compatibility (like Firebase/Android) without blocking the client libraries.
* **Zero Release Bottlenecks**: No sequential staging wait-times; both projects release in parallel.
* **Cons**:
* Relies on external tooling and automation scripts to maintain consistency; human error/drift is still possible if manually edited.