Moves limit and offset methods to postgres syntax

[belchior]

Moves limit and offset methods to postgres syntax

This changes is conceptually a breaking change but in practice is not, because in a only SQL Standard client these methods are not valid and will emit a SQL Syntax Error.

This changes can be a breaking change in non supported scenarios like:

• Uses the library with only standard syntax with a Postgres client
• Uses the library with only standard syntax with a client that support limit and offset

If you are in those scenarios the fix can be

• Enable the postgres feature like

  # Cargo.toml
  
  sql_query_builder = { version = "1.x.x", features = ["postgresql"] }

• Converting the methods to raw like

  // before
  let select = new sql::Select::new()
   .select("*")
   .where_clause("login = $1")
   .limit("100");
  
  // after
  let select = new sql::Select::new()
   .select("*")
   .where_clause("login = $1")
   .raw_after(sql::SelectClause::Where, "LIMITE 100");