ES|QL: SET - define and use cluster level setting in ESQL #138828
Description
Users may often apply the same SET options to most or all queries on a cluster, such as enabling unmapped fields. To reduce boilerplate and improve efficiency, a new (dynamic) esql.query.set.default cluster-wide setting will be introduced.
This setting will define default SET options as key-value pairs, which will be prepended to all ESQL queries. Standard rules of precedence will apply, allowing individual queries to override these global SET settings.
We need to warn users to exercise caution when using global settings, as they can subtly and "magically" alter query semantics. The fact that cluster settings updates require at least manage_cluster_settings privileges should be sufficient.
We will validate that this setting is well formed, but it is acceptable to defer the final validation.