Add PostgreSQL CREATE USER and ALTER USER support

[ramnes]

Currently, the library only supports CREATE ROLE and ALTER ROLE for
PostgreSQL. CREATE USER and ALTER USER fail to parse with errors
like "Expected: an object type after CREATE, found: USER"

But in PostgreSQL reference:

• CREATE USER is equivalent to CREATE ROLE, except that LOGIN is assumed by default
• ALTER USER is an alias to ALTER ROLE
• Both should support the same options as their ROLE counterparts

This commit extends the existing CreateRole and AlterRole
structures to distinct which keyword has been used: USER or
ROLE. It allows these expressions to be parsed and displayed back.

[iffyio]

Could we use something like this representation instead? I think its a somewhat more invasive changes but clearer API wise.

struct CreateUserOrRole {
  names: Vec<ObjectName>,
  if_not_exists: bool,
  // Postgres
  login: Option<bool>,
  inherit: Option<bool>,
  // ...
}

Statement::CreateUser(CreateUserOrRole)
Statement::CreateRole(CreateUserOrRole)

struct AlterUserOrRole {
  name: Ident
}

Statement::AlterUser(AlterUserOrRole)
Statement::AlterRole(AlterUserOrRole)

[ramnes]

We have a namespace problem here, because there's already a CreateUser structure for Snowflake semantics. In the grand scheme of things, CreateUser and CreateRole should probably be both compatible with both Snowflake and PostgreSQL? I'm not sure if I'm seasoned enough with the codebase to bring that in, and that would be a very different kind of work than this PR. :)

[iffyio]

Ah I'm not sure I followed, do we mean snowflake has CreateUser supported by the parser already?

[ramnes]

Yes, but as far as I understand, it's a very different syntax / semantics. Postgres "USER" is really just an alias for "ROLE", and Postgres semantics are currently handled by CreateRole and AlterRole. Which is why I just added the keyword here: it doesn't introduce any backwards compatibility. It's just more data to the AST for an already existing expression parsing.

[ramnes]

Again, this would probably need a refactor so that all dialects use the same structures for ALTER / CREATE USER / ROLE, but I guess it would make more sense in a second step – the inconsistency is already there. This PR fixes a currently broken expression parsing "for free".

[iffyio]

Sorry for the delay in getting back to this. I think ideally we would use separate representations, it would likely be confusing if a CREATE USER statement was being parsed as an CREATE ROLE statement or vice versa by the parser.

Also heads up there is a related PR adding ALTER USER here

[ramnes]

From what I see of that PR, it is modeled on the current CreateUser structure, which only supports Snowflake.

So my understanding is that we'll have CreateUser and AlterUser that work for Snowflake, CreateRole and AlterRole that work for PostgreSQL, and no support for the opposite keyword / technology combo (CREATE USER support on PostgreSQL, or CREATE ROLE support on Snowflake).

I'd be happy to see the four structures unified into something that work for both technologies but I don't think I can do that at the moment, alas. :(