Refactor pinot-common and pinot-core

After extracting the plugins, the next logical step is to clean up pinot-core and pinot-common. Similar to pinot-spi, we should extract all interfaces from common and core into something like pinot-query-spi.

Creating this issue to list all the interfaces we want to extract out.

Here is the list (will keep this up to date based on the comments)

- Plan
- Operator
- Datasource
- Block
- TransformFunction
- AggregationFunction

What else?

Start by reviewing the pinot-common and pinot-core modules and the existing pinot-spi extraction for context. Use the issue's interface list—Plan, Operator, Datasource, Block, TransformFunction, and AggregationFunction—as an initial scope, then resolve what else belongs through the comments. Done requires an agreed extraction plan and implementation of the new pinot-query-spi boundary.

Written by the indexing model from the issue text.