Pass Java SchemaTransform descriptions to Python SDK (#29606)

* pipe thru schematransform descriptions

sdks/java/core/src/main/java/org/apache/beam/sdk/schemas/transforms/SchemaTransformProvider.java

@@ -36,7 +36,11 @@ public interface SchemaTransformProvider {
   /** Returns an id that uniquely represents this transform. */
   String identifier();
 
-  /** Returns a description of this transform to be used for documentation. */
+  /**
+   * Returns a description regarding the {@link SchemaTransform} represented by the {@link
+   * SchemaTransformProvider}. Please keep the language generic (i.e. not specific to any
+   * programming language). The description may be markdown formatted.
+   */
   default String description() {
     return "";
   }

sdks/java/core/src/test/java/org/apache/beam/sdk/schemas/transforms/TypedSchemaTransformProviderTest.java

@@ -61,6 +61,11 @@ public String identifier() {
       return "fake:v1";
     }
 
+    @Override
+    public String description() {
+      return "Description of fake provider";
+    }
+
     @Override
     protected Class<Configuration> configurationClass() {
       return Configuration.class;
@@ -115,6 +120,7 @@ public void testFrom() {
     Configuration outputConfig = ((FakeSchemaTransform) provider.from(inputConfig)).config;
     assertEquals("field1", outputConfig.getField1());
     assertEquals(13, outputConfig.getField2().intValue());
+    assertEquals("Description of fake provider", provider.description());
   }
 
   @Test

sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/providers/BigQueryStorageWriteApiSchemaTransformProvider.java

@@ -98,6 +98,16 @@ public String identifier() {
     return String.format("beam:schematransform:org.apache.beam:bigquery_storage_write:v2");
   }
 
+  @Override
+  public String description() {
+    return String.format(
+        "Writes data to BigQuery using the Storage Write API (https://cloud.google.com/bigquery/docs/write-api)."
+            + "\n\nThis expects a single PCollection of Beam Rows and outputs two dead-letter queues (DLQ) that "
+            + "contain failed rows. The first DLQ has tag [%s] and contains the failed rows. The second DLQ has "
+            + "tag [%s] and contains failed rows and along with their respective errors.",
+        FAILED_ROWS_TAG, FAILED_ROWS_WITH_ERRORS_TAG);
+  }
+
   @Override
   public List<String> inputCollectionNames() {
     return Collections.singletonList(INPUT_ROWS_TAG);