Make shardsvr mongods also act as a sampler for query sampling

$lookup, $graphLookup and $unionWith stages take in a "pipeline" that can be executed against any collection. Such nested aggregate queries do not go through mongoses since they are routed by the shardsvr mongods themselves. To support sampling such queries, we need to allow shardsvr mongods to act as a sampler (i.e. QueryAnalysisSampler).