Introduce entry-based DSL for JdbcClient batch updates

Prior to this commit, JdbcClient's BatchSpec bound parameters to a
"current" batch entry and separated consecutive entries via add(). That
style was error-prone, since a missing add() still compiled but
silently merged parameters into a single entry. It also did not lend
itself to providing batch parameters dynamically — for example, from an
existing collection of domain objects.

To address that, this commit replaces the chaining style with
self-contained batch entries. BatchSpec now provides entry(...) methods
that define a single entry — via a Consumer<BatchEntry> callback, a
List of positional parameters, or a Map of named parameters — as well
as entries(...) methods that define one entry per parameter object.
Such calls can be freely combined within the same batch.

The new BatchEntry type exposes only the singular param(...) and
paramSource(...) methods. The add() method as well as the param(...),
params(...), and paramSource(...) methods have been removed from
BatchSpec.

Closes gh-37341

framework-docs/modules/ROOT/pages/data-access/jdbc/core.adoc

@@ -679,17 +679,30 @@ provides `firstName` and `lastName` properties, such as the `Actor` class from a
 ----
 
 For batch updates, accumulate the batch entries in a fluent fashion through `batch()`,
-binding the parameters for each entry as for a single update – either as positional
-parameters or as named parameters – and separating consecutive entries with `add()`.
-The accumulated entries are executed as a single JDBC batch by `update()`, which
-also completes the final entry implicitly:
+defining each entry through `entry(...)` – either with a callback that binds positional
+or named parameters as for a single update, or with a `List` of positional parameters
+or a `Map` of named parameters. The accumulated entries are executed as a single JDBC
+batch by `update()`:
 
 [source,java,indent=0,subs="verbatim,quotes"]
 ----
 	this.jdbcClient.sql("insert into t_actor (first_name, last_name) values (:firstName, :lastName)")
 			.batch()
-				.param("firstName", "Leonor").param("lastName", "Watling").add()
-				.param("firstName", "Christian").param("lastName", "Bale")
+				.entry(entry -> entry.param("firstName", "Leonor").param("lastName", "Watling"))
+				.entry(entry -> entry.param("firstName", "Christian").param("lastName", "Bale"))
+			.update();
+----
+
+Alternatively, define several entries at once through `entries(...)`, with each given
+parameter source object – for example, an `Actor` record – representing one entry.
+`entry(...)` and `entries(...)` calls may be freely combined within the same batch:
+
+[source,java,indent=0,subs="verbatim,quotes"]
+----
+	List<Actor> actors = ...;
+	this.jdbcClient.sql("insert into t_actor (first_name, last_name) values (:firstName, :lastName)")
+			.batch()
+				.entries(actors)
 			.update();
 ----