[mongo] add config option to enable search indexes collection

[lu-zhengda]

What does this PR do?

This PR introduces a configuration option to control the collection of MongoDB Atlas search indexes via the $listSearchIndexes aggregation pipeline. By default, this feature is now disabled.

Motivation

The change is motivated by performance considerations, as the $listSearchIndexes aggregation can run slowly and impact database performance.

https://datadoghq.atlassian.net/browse/DBMON-4910

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.56%. Comparing base (c31d8c2) to head (4920dad).
Report is 7 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
hive	?
hivemq	?
hudi	?
ignite	?
jboss_wildfly	?
kafka	?
mongo	93.00% <100.00%> (-0.81%)	⬇️
presto	?
solr	?

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