[SPARK-21778][SQL] Simpler Dataset.sample API in Scala / Java

## What changes were proposed in this pull request?
Dataset.sample requires a boolean flag withReplacement as the first argument. However, most of the time users simply want to sample some records without replacement. This ticket introduces a new sample function that simply takes in the fraction and seed.

## How was this patch tested?
Tested manually. Not sure yet if we should add a test case for just this wrapper ...

Author: Reynold Xin <rxin@databricks.com>

Closes #18988 from rxin/SPARK-21778.

sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala

@@ -1848,11 +1848,43 @@ class Dataset[T] private[sql](
     Except(logicalPlan, other.logicalPlan)
   }
 
+  /**
+   * Returns a new [[Dataset]] by sampling a fraction of rows (without replacement),
+   * using a user-supplied seed.
+   *
+   * @param fraction Fraction of rows to generate, range [0.0, 1.0].
+   * @param seed Seed for sampling.
+   *
+   * @note This is NOT guaranteed to provide exactly the fraction of the count
+   * of the given [[Dataset]].
+   *
+   * @group typedrel
+   * @since 2.3.0
+   */
+  def sample(fraction: Double, seed: Long): Dataset[T] = {
+    sample(withReplacement = false, fraction = fraction, seed = seed)
+  }
+
+  /**
+   * Returns a new [[Dataset]] by sampling a fraction of rows (without replacement).
+   *
+   * @param fraction Fraction of rows to generate, range [0.0, 1.0].
+   *
+   * @note This is NOT guaranteed to provide exactly the fraction of the count
+   * of the given [[Dataset]].
+   *
+   * @group typedrel
+   * @since 2.3.0
+   */
+  def sample(fraction: Double): Dataset[T] = {
+    sample(withReplacement = false, fraction = fraction)
+  }
+
   /**
    * Returns a new [[Dataset]] by sampling a fraction of rows, using a user-supplied seed.
    *
    * @param withReplacement Sample with replacement or not.
-   * @param fraction Fraction of rows to generate.
+   * @param fraction Fraction of rows to generate, range [0.0, 1.0].
    * @param seed Seed for sampling.
    *
    * @note This is NOT guaranteed to provide exactly the fraction of the count
@@ -1871,7 +1903,7 @@ class Dataset[T] private[sql](
    * Returns a new [[Dataset]] by sampling a fraction of rows, using a random seed.
    *
    * @param withReplacement Sample with replacement or not.
-   * @param fraction Fraction of rows to generate.
+   * @param fraction Fraction of rows to generate, range [0.0, 1.0].
    *
    * @note This is NOT guaranteed to provide exactly the fraction of the total count
    * of the given [[Dataset]].