Implementation:
* Added a new field allow_readonly to table system.table_functions.
* Updated to use new field allow_readonly to allow using table functions in readonly mode.
* Added TableFunctionProperties similar to AggregateFunctionProperties.
* The functions allowed in readonly mode are in this set table_functions_allowed_in_readonly_mode.
Testing:
* Added a test for filesystem tests/queries/0_stateless/02473_functions_in_readonly_mode.sh
Documentation:
* Updated the english documentation for Table Functions.