[SPARK-30689][CORE][FOLLOW-UP] Rename config name of discovery plugin

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

Rename config `spark.resources.discovery.plugin` to `spark.resources.discoveryPlugin`.

Also, as a side minor change: labeled `ResourceDiscoveryScriptPlugin` as `DeveloperApi` since it's not for end user.

### Why are the changes needed?

Discovery plugin doesn't need to reserve the "discovery" namespace here and it's more consistent with the interface name `ResourceDiscoveryPlugin` if we use `discoveryPlugin` instead.

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

No, it's newly added in Spark3.0.

### How was this patch tested?

Pass Jenkins.

Closes apache#27689 from Ngone51/spark_30689_followup.

Authored-by: yi.wu <yi.wu@databricks.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

core/src/main/scala/org/apache/spark/internal/config/package.scala

@@ -55,7 +55,7 @@ package object config {
       .createOptional
 
   private[spark] val RESOURCES_DISCOVERY_PLUGIN =
-    ConfigBuilder("spark.resources.discovery.plugin")
+    ConfigBuilder("spark.resources.discoveryPlugin")
       .doc("Comma-separated list of class names implementing" +
         "org.apache.spark.api.resource.ResourceDiscoveryPlugin to load into the application." +
         "This is for advanced users to replace the resource discovery class with a " +

core/src/main/scala/org/apache/spark/resource/ResourceDiscoveryScriptPlugin.scala

@@ -21,6 +21,7 @@ import java.io.File
 import java.util.Optional
 
 import org.apache.spark.{SparkConf, SparkException}
+import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.api.resource.ResourceDiscoveryPlugin
 import org.apache.spark.internal.Logging
 import org.apache.spark.util.Utils.executeAndGetOutput
@@ -32,6 +33,7 @@ import org.apache.spark.util.Utils.executeAndGetOutput
  * If the user specifies custom plugins, this is the last one to be executed and
  * throws if the resource isn't discovered.
  */
+@DeveloperApi
 class ResourceDiscoveryScriptPlugin extends ResourceDiscoveryPlugin with Logging {
   override def discoverResource(
       request: ResourceRequest,

docs/configuration.md

@@ -244,7 +244,7 @@ of the most common options to set are:
   </td>
 </tr>
 <tr>
- <td><code>spark.resources.discovery.plugin</code></td>
+ <td><code>spark.resources.discoveryPlugin</code></td>
   <td>org.apache.spark.resource.ResourceDiscoveryScriptPlugin</td>
   <td>
     Comma-separated list of class names implementing