Move validation for InsertInto nodes into the analyzer's validation phase

[fulghum]

The resolveInsertRows analyzer rule contains various validation checks for InsertInto plan nodes. A customer reported a panic in dolthub/dolt#7313 because an InsertInto validation error wasn't caught before the assignExecIndexes rule ran. Moving the validation from resolveInsertRows up into a new rule in the validation batch catches the validation error before the assignExecIndexes rule runs and prevents the panic the customer hit.

The only difference I see in this refactoring is that the new validateInserts rule doesn't invoke the analyzer on InsertInto.Source when the source is a TriggerExecutor. This is still done in the resolveInsertRows rule and the result is used to transform the node being processed, but the functions used by the new validation rule (i.e existsNonZeroValueCount, validateColumns, validateValueCounts) don't seem to actually need the TriggerExecutor to be analyzed independently yet at that point.

Related to: dolthub/dolt#7313

[max-hoffman]

closing in favor of #2286