QL: Support runtime-fields at query-time

[costin]

With the introduction of query-time runtime fields, requests can be enhanced with "runtime"/"virtual" fields that augment the mapping.
Both declarative languages should allow clients to specify such fields if desired, similar to the filter parameter.
As a side-note, as the number of params increases, it might make sense to encapsulate ES query parameter into a dedicated object so potential future additions, don't end up clashing with other top-level params:

filter: {} -> enrich { filter: {} , runtime-mapping : {}, <extra-option> : {}

[elasticmachine]

Pinging @elastic/es-ql (Team:QL)

[astefan]

This is implemented with #71356.