(dsl): Implement exists query

modules/library/src/main/scala/zio/elasticsearch/ElasticQuery.scala

@@ -43,6 +43,8 @@ object ElasticQuery {
 
   def boolQuery(): BoolQuery = BoolQuery.empty
 
+  def exists(field: String): Exists = Exists(field)
+
   def range(field: String): Range[Unbounded.type, Unbounded.type] = Range.empty(field)
 
   private[elasticsearch] final case class BoolQuery(must: List[ElasticQuery], should: List[ElasticQuery])
@@ -62,6 +64,10 @@ object ElasticQuery {
     def empty: BoolQuery = BoolQuery(Nil, Nil)
   }
 
+  private[elasticsearch] final case class Exists private (field: String) extends ElasticQuery {
+    override def toJson: Json = Obj("exists" -> Obj("field" -> field.toJson))
+  }
+
   private[elasticsearch] final case class Match[A: ElasticPrimitive](field: String, value: A) extends ElasticQuery {
     override def toJson: Json = Obj("match" -> Obj(field -> value.toJson))
   }

modules/library/src/test/scala/zio/elasticsearch/QueryDSLSpec.scala

@@ -89,6 +89,11 @@ object QueryDSLSpec extends ZIOSpecDefault {
             )
           )
         },
+        test("successfully create Exists Query") {
+          val query = exists(field = "day_of_week")
+
+          assert(query)(equalTo(Exists(field = "day_of_week")))
+        },
         test("successfully create empty Range Query") {
           val query = range(field = "customer_age")
 
@@ -210,6 +215,21 @@ object QueryDSLSpec extends ZIOSpecDefault {
 
           assert(query.toJsonBody)(equalTo(expected.toJson))
         },
+        test("properly encode Exists Query") {
+          val query = exists(field = "day_of_week")
+          val expected =
+            """
+              |{
+              |  "query": {
+              |    "exists": {
+              |      "field": "day_of_week"
+              |    }
+              |  }
+              |}
+              |""".stripMargin
+
+          assert(query.toJsonBody)(equalTo(expected.toJson))
+        },
         test("properly encode Unbounded Range Query") {
           val query = range(field = "field")
           val expected =