Added support for DSL Exists query to convert in RelNode format

[abhishek00159]

Description

Implements the QueryTranslator interface to convert OpenSearch ExistsQueryBuilder to Calcite IS NOT NULL RexNode.

1. Created ExistsQueryTranslator

Key Logic:

• Extracts field name from ExistsQueryBuilder
• Looks up field in schema to get field index and throw exception if boost is passed with not default value or field is not passed.
• Creates IS NOT NULL($fieldIndex) RexNode using Calcite's SqlStdOperatorTable.IS_NOT_NULL

Calcite Output:

LogicalFilter(condition=[IS NOT NULL($N)])
  LogicalTableScan(table=[[index-name]])

2. Registered ExistsQueryTranslator

File: src/main/java/org/opensearch/dsl/query/QueryRegistryFactory.java

Added registration in the create() method:

registry.register(new ExistsQueryTranslator());

This makes the translator available for all exists queries.

3. Added Integration Tests

File: src/internalClusterTest/java/org/opensearch/dsl/DslLogicalPlanIntegrationIT.java

Added three comprehensive tests.

1. Exists query test
2. Exists query with bool.
3. Exist query not supported for boost.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]
NA

Check List

• [Done ] Functionality includes testing.
• [NA ] API changes companion pull request created, if applicable.
• [ NA] Public documentation issue/PR created, if applicable.

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[nssuresh2007]

LGTM