perf: decode values directly from row

[olavloite]

Remove the intermediate step of first decoding a value to a GenericColumnValue, and then into the Go type that should be returned. This reduces the number of memory allocations needed to decode query results.

This is also needed for implementing https://pkg.go.dev/database/sql/driver#RowsColumnTypeScanType.

BenchmarkRows_Next_New-8        	     895	   1305352 ns/op	 1198360 B/op	   89750 allocs/op
BenchmarkRows_Next_Proto-8       	    1179	    991260 ns/op	  960408 B/op	   60006 allocs/op
BenchmarkRows_Next_Old-8   	             649	   1774592 ns/op	 1678361 B/op	  119750 allocs/op

(Note: The 'proto' option in the above table is the option where the row is not decoded at all, but just returns the proto value. That part was not changed in this pull request.)

[gemini-code-assist]

Code Review

This pull request optimizes the Next method in rows.go by caching column types and utilizing direct column decoding, which minimizes the creation of intermediate GenericColumnValue objects. A potential nil pointer dereference was identified in the Next method if r.colTypes is accessed after a failed metadata initialization; it is recommended to add a check to handle this state safely.