User feedback on REALTIME table update for topic modification

[MrNeocore]

Following Slack discussion : https://apache-pinot.slack.com/archives/C011C9JHN7R/p1622809155406800

Changing the topic a REALTIME table is consuming from is not possible as for now.
-> stream.kafka.topic.name

However, a table definition update is fully accepted without warnings.

We should provide some user feedback regarding this limitation.

1. In the docs: I'll open a PR.
2. At the API level: We could either warn or fully reject the table update in such instances.

[mcvsubbu]

We had considered this before. The best way to do this seemed to be to introduce a new API call into streams, returning a boolean whether or not the new streams config is compatible with the old one. Each underlying stream would then have to implement the logic to scan through. In case of Kafka, change in topic, or change in broker address should not be permitted (at least for some implementations). Some implementations may allow change in broker address as long as the topic is the same.

For Kinesis, appropriate checks need to be added.

Thanks for filing the issue.