Configuration to disable opensearch heap warnings, added tests

[todvora]

Description

Added datanode configuration key data_node_heap_size_warning_enabled (default true), to skip the heap size warning check and possible warning notification.

Motivation and Context

Implements https://github.com/Graylog2/graylog-plugin-enterprise/issues/12164

How Has This Been Tested?

Added unit tests

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.