new table function: sqlrs_functions

[Fedomn]

Currently, we already have sqlrs_tables() and sqlrs_columns table function.

https://github.com/Fedomn/sqlrs/blob/0bfe264000787201e4dfdea4c9079373f33f344a/src/function/table/sqlrs_tables.rs#L118-L122

Then, we should implement more internal metadata table, such as:

• sqlrs_functions()