Introduce a feature flag to set the default value of cluster.logsdb.enabled

[salvatore-campagna]

Description

PR #109025 introduced a node level settings which controls whether the logs index mode is used by default in logs@settings. Introducing the index mode in the existing logs@settings composable template results in indices
matching the logs-*-* pattern to adopt the new logs index mode.

Enabling the index mode by default, anyway, still requires us to fix a couple of issues with synthetic source first:

• Support ignore_malformed for all non-string fields using synthetic source #106483
• Synthetic source failure when doc_values: false #109546

Fixing such issues is required to prevent some tests failures, including:

• org.elasticsearch.datastreams.LogsDataStreamIT
• org.elasticsearch.datastreams.EcsLogsDataStreamIT

and to prevent mapping failures in the elastic/logs Rally nightly track execution.

For this reason the setting was created with a default value of false and the feature flag introduction has
been postponed waiting for such issues to be fixed. After fixing the issues above we can proceed with
using a feature flag to set the default value of cluster.logsdb.enable to true for snapshot builds only.

Before merging the PR including the feature flag, anyway, we need to make sure we address any Rally issue so that we can prevent failures of the nightly benchmark of elastic/logs. See #109523.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[salvatore-campagna]

We are considering not doing this anymore for two reasons:

1. enabling LogsDB will already need an override for the setting and upgrading of the version of the stack template registry. Adding a feature flag seems something overkilling, making things even more complicated.
2. even without a feature flag we have a setting that integrations can act on to enable/disabled LogsDB. Sure not having the feature flag does not give us snapshot builds with LogsDB by default, but that is not really an obstacle when it comes to test integrations.

Anyway, not matter if we decide or not about adding the feature flag, if we decide to have it, we will add it after the integration testing activity is complete so that the feature flag is available when LogsDB is in Tech Preview and not before.