[Feature] Add task:set_metric_action lua API method

[Feature] Add task:set_metric_action lua API method

rspamd_config:set_metric_symbol({table})

Set the value of a specified symbol in a metric.
This function accepts table with the following elements:

- `name`: name of symbol (string)
- `score`: score for symbol (number)
- `metric`: name of metric (string, optional)
- `description`: description of symbol (string, optional)
- `group`: name of group for symbol (string, optional)
- `one_shot`: turn off multiple hits for a symbol (boolean, optional)
- `flags`: comma separated string of flags:
   + `ignore`: do not strictly check validity of symbol and corresponding rule
   + `one_shot`: turn off multiple hits for a symbol
- `priority`: priority of symbol's definition