Allow mapping authentication query columns to user attributes

[jnbdz]

The SQL authentication provider discards every column of the authentication query except the password (row.getString(0)), so callers that need more user data right after login — typically the user id for a session or token — have to run a second query for the row they just fetched.

Following the guidance in #694, this adds a SqlAuthentication.create(client, options, attributeMapper) overload where the mapper is a Function<Row, JsonObject> living on the auth type (options stay data-only). The mapper receives the authenticated row and the returned JSON object is merged into the authenticated User attributes:

AuthenticationProvider provider = SqlAuthentication.create(sqlClient,
  new SqlAuthenticationOptions()
    .setAuthenticationQuery("SELECT password, email FROM users WHERE username = ?"),
  row -> new JsonObject().put("email", row.getString("email")));

Behavior notes:

• the password is still expected in the first column, existing create overloads are unchanged
• a null mapper result is ignored
• a mapper that throws fails the authentication future
• the mapper only runs after the password has been verified

Includes a test in MySQLTest (the DDL gains a nullable email column), an example and documentation.

Fixes #694