[WIP] add SourcePage interface for delayed materialization of ConnectorSourceData

[dain]

Warning

THIS IS A WORK IN PROGRESS

Description

This adds a new interface to the SPI, SourcePage, which will be the eventual replacement for Page in ConnectorPageSource. Since SourcePage is an interface it allows the connector to directly know when columns are being accessed.

Additionally, SourcePage is not intended to be thread safe, it can be mutable. Specifically, the interface contains the method:

void selectPositions(int[] positions, int offset, int size);

This reduces the positions that will be returned from the SourcePage, and since this is a mutation operation the connector knows that only the specified positions can be accessed. This allows data sources to use this information for skipping unnecessary reads.

Release notes

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## Section
* Fix some things. ({issue}`issuenumber`)