Support PostgreSQL SCRAM-SHA-256

[solardiz]

There's apparently a common kind of encoding for SCRAM verifiers, which we don't support yet:

https://github.com/search?q=SCRAM-SHA-256%24&type=code

// AsPasswordString returns the password as defined in https://www.postgresql.org/docs/15/catalog-pg-authid.html
func (password ScramSha256Password) AsPasswordString() string {
	return fmt.Sprintf(`SCRAM-SHA-256$%d:%s$%s:%s`,
		password.Iterations, password.Salt.ToBase64(), password.StoredKey.ToBase64(), password.ServerKey.ToBase64())

https://github.com/svenvc/P3/blob/master/P3/P3SCRAM.class.st

About PostgreSQL's password storage 

If you execute 'SELECT rolname,rolpassword FROM pg_authid' with enough privileges,
you can see how PostgreSQL stores passwords. In the case of SCRAM-SHA-256, 
rolpassword seems to have the following format (using terms used in this implementation): 

SCRAM-SHA-256$<iterations>:<salt(base64>$<storedKey(base64>:<serverKey(base64)>

To support the colons, we need a prepare method that would turn these strings into what can go into john.pot, or a 2john script.

[solardiz]

To support the colons, we need a prepare method that would turn these strings into what can go into john.pot, or a 2john script.

To be clear, I think prepare is the better way to go, and a 2john script is a shortcut or fallback approach. @kholia