### Rationale for this change
Static Arrow linking can fail for bundled GCS and system Abseil.
### What changes are included in this PR?
Export Abseil dependencies via CMake target and pkg-config metadata.
### Are these changes tested?
To be additionally verified by CI.
### Are there any user-facing changes?
This improves downstream static linking.
### Was AI used for this PR?
**PR code and description written by:**
- [X] Human
- [X] AI
**Reviewed before submission by:**
- [X] Human
- [X] AI
- [ ] Not reviewed
* GitHub Issue: #51651
Authored-by: Tadeja Kadunc <tadeja.kadunc@gmail.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>