Add shard_size for Terms aggregations

[gmoskovicz]

Seems to be related to: #922.

Currently there is no way to set the shard size, other than using the Advanced Json Input. It would be great if under advanced we have a shard size as well. This will make it easier to set the Shard Size. And we could use the same input type to allow just numbers.

[thomasneirynck]

@gmoskovicz this got quite a few upvotes in a very short time, so where there's smoke.... ;) Are you guys running into a particular problem without having this option? Anything in specific that triggered your request? thx

[gmoskovicz]

@thomasneirynck we just had a conversation with a customer. We just said to use an ES Client node in front of Kibana. But i think that it might be better to do what Logstash does, which is what this is described.

They wanted to point Kibana to multiple nodes, which is not possible as we know.

I know that it's not a frequent ask, but worth keeping it for the future.

[ghudgins]

We plan on delivering this in this issue - #117909