Support user-defined variables.
After this PR, we can use `set @a = xx` to define a user variable and use it in the query like `select @a`.
the changes of this PR:
1. Support the grammar for `set user variable` in the parser.
2. Add the `userVars` in `VariableMgr` to store the user-defined variables.
3. For the `set @a = xx`, we will store the variable name and its value in the `userVars` in `VariableMgr`.
4. For the `select @a`, we will get the value for the variable name in `userVars`.
