[backend] Add an application setting for enabling the slowlog

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Background

Enabling the slowlog is manually done using change requests. This doesn't scale and the team needs to find an automated way to do this.

Proposal

• Add a setting for each index to enable the slowlog or not.
• The index settings are stored in Elastic::IndexSetting
• add a boolean column for enable_slowlog that defaults to false with a not null constraint
• this setting should be respected anytime the index is created from scratch or recreated during zero downtime reindexing or reindexing the instance