Add Criteria example for nested Elasticsearch field

[parazit-IR]

Relates to #591.

This PR adds a small Spring Data Elasticsearch example that demonstrates using
org.springframework.data.elasticsearch.core.query.Criteria against a nested field.

Changes:

• Introduced a Talk document with a speakers field annotated as @Field(type = FieldType.Nested).
• Added a simple Speaker value object.
• Extended ElasticsearchOperationsTest with criteriaQueryOnNestedSpeakers(), which:
  • saves one Talk with a nested Speaker,
  • saves another Talk without speakers,
  • runs a CriteriaQuery on speakers.name,
  • asserts that only the matching talk is returned.

Note:
On my local machine, the existing ElasticsearchOperationsTest fails to start the
ApplicationContext with a DataAccessResourceFailureException when initializing ConferenceRepository:

status: 400, [es/indices.exists] Expecting a response body, but none was sent.

This also happens on a clean checkout of main.
The changes in this PR are limited to adding the nested Criteria example and do not modify
the existing Elasticsearch configuration.