database/sql: allow drivers to override Scan behavior

[jackc]

Implementing RowsColumnScanner allows the driver
to completely control how values are scanned.

Fixes #67546

[gopherbot]

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 1: Hold+1

(1 comment)

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[gopherbot]

This PR (HEAD: 4504997) has been imported to Gerrit for code review.

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