[FEATURE] Support Multi search template API

[gquintillan]

It would be nice if org.springframework.data.elasticsearch.core.SearchOperations#multiSearch(java.util.List<? extends org.springframework.data.elasticsearch.core.query.Query>, java.lang.Class<T>) supports Multi Search Templates.

At the moment It is possible to use org.springframework.data.elasticsearch.core.query.SearchTemplateQuery like this:

private final ElasticsearchOperations elasticsearchOperations;

[...]

SearchTemplateQuery searchQueryA = SearchTemplateQuery.builder()
                .withId("search_query_template_id")
                .withParams(
                        Map.of(
                                "param_a", "value")))
                .build();
elasticsearchOperations.search(searchQueryA, CustomDoc.class);

But it would be nice to also be able to run Multi Search Templates like this:

private final ElasticsearchOperations elasticsearchOperations;

[...]

SearchTemplateQuery searchQueryA = SearchTemplateQuery.builder()
                .withId("search_query_template_id_a")
                .withParams(
                        Map.of(
                                "param_a", "value")))
                .build();

SearchTemplateQuery searchQueryB = SearchTemplateQuery.builder()
                .withId("search_query_template_id_b")
                .withParams(
                        Map.of(
                                "param_a", "value")))
                .build();

elasticsearchOperations.multiSearch(List.of(searchQueryA, searchQueryB), CustomDoc.class);

More info: https://www.elastic.co/guide/en/elasticsearch/reference/current/multi-search-template.html

[gquintillan]

I tried this alternative:

private final ElasticsearchClient elasticsearchClient;
[...]

final MsearchTemplateRequest msearchTemplateRequest = MsearchTemplateRequest.of([...]);

[...]

elasticsearchClient.msearchTemplate(msearchTemplateRequest, CustomDoc.class);

But the annotation@Document is not taken into account (it does not map the document to the POJO as wanted) and we lose very useful features that are included with ElasticsearchOperations.

[sothawo]

elasticsearchOperations.multiSearch(List.of(searchQueryA, searchQueryB), CustomDoc.class);

would probably be not enought, each of the queries could target a different index and might have a different entity class as it's return value. So we'd need a list of entity classes as second argument.
Basically the same emthod variants as we have for ElasticsearchOperations.multiSearch().