[java-cloud-bom] Suggestion to add management of com.google.re2j:re2j

**Is your feature request related to a problem? Please describe.**

We are using the `com.google.cloud:libraries-bom` (through `com.google.cloud:spring-cloud-gcp-dependencies`) to manage versions for our Google Cloud dependencies. We've noticed that `com.google.re2j:re2j` is included as a transitive runtime dependency of `com.google.cloud:google-cloud-spanner`, but its version is not managed in the libraries-bom.

We have validations that our release build resolves to one single version of each dependency, and this dependency is also included through another of our dependencies in another version. This necessitates that we manage this dependency ourselves. We would prefer if this dependency would be managed through the import of the Google BOM (being a Google dependency, just like Guava for example).

**Describe the solution you'd like**

That dependency management of `com.google.re2j:re2j` would be included through the import of `com.google.cloud:libraries-bom`.

**Describe alternatives you've considered**

We can (and do) manage this version by ourselves, but would prefer if this managed was handled through this BOM import instead.