[java-spanner] Truncating and Limiting the Value's class toString() to 36 characters

In java class [Value](https://github.com/googleapis/java-spanner/blob/main/google-cloud-spanner/src/main/java/com/google/cloud/spanner/Value.java) for 3 data types which includes `String`, `JSON`, `PGJSONB` we are truncating the response of method `valueToString` to 36 characters. Method `valueToString` is called internally by `toString()` method.

The `toString()` method is designed to return a string representation of an object, and it's expected to provide a complete and meaningful representation of the object's state. Truncating it may lead to loss of information and make debugging and logging more difficult.