### Description

`TransportFieldCapabilitiesAction` (used for the the `_field_caps` endpoint) has got a nested loop in `merge` method when unmapped fields are included (https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/action/fieldcaps/TransportFieldCapabilitiesAction.java#L245). This loops over indices -> fields -> indices, which could cause an exponential blowup in objects used when there's a large number of indices and unmapped fields.

We could be more efficient in creating & sharing IndexCap objects in these situations.
server/src/main/java/org/elasticsearch/action/fieldcaps/FieldCapabilities.java
name,
            type,
            isMetadataField,
            isSearchable,
            isAggregatable,
            false,
            null,
            indices,
            nonSearchableIndices,
            nonAggregatableIndices,
            null,
            null,
            meta
        );
    }
    /**
     * Constructor for a set of indices used by parser
     * @param name The name of the field
     * @param type The type associated with the field.
     * @param isMetadataField Whether this field is a metadata field.
     * @param isSearchable Whether this field is indexed for search.
     * @param isAggregatable Whether this field can be aggregated on.
     * @param isDimension Whether this field can be used as dimension
     * @param metricType If this field is a metric field, returns the metric's type or null for non-metrics fields
     * @param indices The list of indices where this field name is defined as {@code type},
     *                or null if all indices have the same {@code type} for the field.
     * @param nonSearchableIndices The list of indices where this field is not searchable,
     *                             or null if the field is searchable in all indices.
     * @param nonAggregatableIndices The list of indices where this field is not aggregatable,
     *                               or null if the field is aggregatable in all indices.