Migrate OpenSearchBackend and related search functionality to the new Java client

[moesterheld]

Description

Needed to be done in one go due to the necessary interface method signatures in OSGeneratedQueryContext.

Major changes:

• Builders in the new client are immutable and write-only. For the SearchRequest.Builder, a wrapper MutableSearchRequestBuilder had to be introduced (see javadoc)
• Aggregations in the new client are unnamed and set using a Map<String, Aggregation>. This needed some adjustments.

/nocl internal change

/jpd https://github.com/Graylog2/graylog-plugin-enterprise/pull/13152

Motivation and Context

part of https://github.com/Graylog2/graylog-plugin-enterprise/issues/12465

How Has This Been Tested?

existing tests, running Graylog locally

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.