### 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
this.meta = Objects.requireNonNull(meta);
    }
    /**
     * Constructor for non-timeseries field caps. Useful for testing
     * Constructor for a set of indices.
     * @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 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.
     * @param meta Merged metadata across indices.
     */
    public FieldCapabilities(
        String name,
        String type,
        boolean isMetadataField,
        boolean isSearchable,
        boolean isAggregatable,
        String[] indices,
        String[] nonSearchableIndices,
        String[] nonAggregatableIndices,
        Map<String, Set<String>> meta
    ) {
        this(