Incorrect enumeration mappings

[paulourio]

There are several enums with incorrect mappings in Go.

At /zetasql/parser/ast_enums.proto we types such as ASTWindowFrameEnums and several others that actually start at 1.
They all have a comment "0 reserved for future".
Meanwhile, all definitions in Go start at zero, so many of these enums are shifted by one: for ASTWindowFrameEnums, in Go UnboundedPrecedingType has value 0 but it should be 1.