[SPARK-53457] Expose fabric8 dependencies as api for downstream users

### What changes were proposed in this pull request?

This PR exposes fabric8 dependencies as api in gradle build and hence in pom

### Why are the changes needed?

Our public API surfaces fabric8 types (e.g. PodTemplateSpec), so hiding the dependencies behind `implementation` may cause comile errors for customers who need to use those types along with our API library.

This enhances developer experience and avoid surprises when integrating our API with projects that already depends on fabric8.

### Does this PR introduce _any_ user-facing change?

Pom change only

### How was this patch tested?

CIs

### Was this patch authored or co-authored using generative AI tooling?

No

Author: jiangzho

spark-operator-api/build.gradle

@@ -16,9 +16,13 @@
  * specific language governing permissions and limitations
  * under the License.
  */
+plugins {
+  id 'java-library'
+}
+
 dependencies {
   // fabric8
-  implementation(libs.kubernetes.client) {
+  api(libs.kubernetes.client) {
     exclude group: 'com.squareup.okhttp3'
   }
   compileOnly(libs.crd.generator.apt)