**Elasticsearch version** (`bin/elasticsearch --version`): 7.16 (earlier too), 8.0, 8.1

**Description of the problem including expected versus actual behavior**:
Currently, the rollover requests are not cancellable, irrespective of the underlying incoming HTTP channel being closed (eg. by a proxy). This could result in clients receiving `504` and retrying the rollovers, resulting in multiple rollover operations being processed (some products issue rollovers without specifying conditions, so this could translate in multiple subsequent rollovers being executed).

This proposes using the `RestCancellableNodeClient` infrastructure for the rollover API, such that elasticsearch can react to the incoming HTTP connection being closed by cancelling the rollover operation.

server/src/main/java/org/elasticsearch/action/admin/indices/rollover/RolloverRequest.java
MaxPrimaryShardDocsCondition maxPrimaryShardDocsCondition = new MaxPrimaryShardDocsCondition(numDocs);
        if (this.conditions.containsKey(maxPrimaryShardDocsCondition.name)) {
            throw new IllegalArgumentException(maxPrimaryShardDocsCondition.name + " condition is already set");
        }
        this.conditions.put(maxPrimaryShardDocsCondition.name, maxPrimaryShardDocsCondition);
    }
    public boolean isDryRun() {
        return dryRun;
    }
    public Map<String, Condition<?>> getConditions() {
        return conditions;
    }
    public String getRolloverTarget() {
        return rolloverTarget;
    }
    public String getNewIndexName() {
        return newIndexName;
    }
    /**
     * Returns the inner {@link CreateIndexRequest}. Allows to configure mappings, settings and aliases for the new index.
     */
    public CreateIndexRequest getCreateIndexRequest() {
        return createIndexRequest;
    }
    
    public void fromXContent(boolean isTypeIncluded, XContentParser parser) throws IOException {
        PARSER.parse(parser, this, isTypeIncluded);
    }
}