feat: add global OpenSearch index prefix support (#8489)

[mezzeddinee]

Fixes #8489 by introducing index prefixing as a global OpenSearch setting and removing MonitoringDB-local prefix handling.

What changed

• src/DIRAC/ConfigurationSystem/Client/Utilities.py

  • Read /Systems/NoSQLDatabases/IndexPrefix in getElasticDBParameters.
  • Normalize value (strip().lower()) and pass it as IndexPrefix.

• src/DIRAC/Core/Base/ElasticDB.py

  • Read global IndexPrefix from DB parameters.
  • Forward it to ElasticSearchDB as globalIndexPrefix.

• src/DIRAC/Core/Utilities/ElasticSearchDB.py

  • Added globalIndexPrefix constructor parameter and property setter.
  • Added _withGlobalPrefix() helper to prepend the global prefix to index names/patterns.
  • Applied prefixing across index/template/search/read/write/delete operations.
  • Refactored bulk_index flow to avoid apparent double-prepending.

• src/DIRAC/MonitoringSystem/DB/MonitoringDB.py

  • Removed MonitoringDB-specific IndexPrefix lookup from CS.
  • Rely on inherited global prefix mechanism via ElasticDB.

• docs/source/AdministratorGuide/ExternalsSupport/index.rst

  • Documented IndexPrefix under Systems/NoSQLDatabases and clarified it is lower-case.

• docs/source/AdministratorGuide/Systems/MonitoringSystem/index.rst

  • Updated docs to reference global Systems/NoSQLDatabases/IndexPrefix.

• src/DIRAC/Core/scripts/install_full.cfg

  • Added commented example for global IndexPrefix under NoSQLDatabases.