pgerror: use a different type for pgcodes

[knz]

tldr: we want to change the API signature of pgerror.Newf and pgerror.Wrapf and possibly other things that accept a pg error code as argument.

Background

As found in #49660, there was a mistake in the code

  stmt := "some string"
  return pgerror.Wrapf(err, "while running %s", stmt)

The second argument should be the pgcode, not the error string.

The Go type system does not catch this - the interface of Wrapf is (error, string, string, ...interface{})

It's too easy to mistakenly forget the pgcode in there.

We can't use type Code string or type Code = string because that doesn't block the error above.

Proposal

We can do this in package pgcode:

type Code struct {
  code string
}

func MakeCode(c string) Code { return Code{c} }

func (c Code) String() string { return c.code }

then change pgerror.Wrapf to take (error, pgcode.Code, string, ...interface{})

All the pre-defined code can be generated to use that struct instead of a simple string (the're a generation script already, so we can automate the conversion).

suggested by @dt @tbg