[Feature Request] Add ability to specify alert handlers per alert.

Please allow users to add handlers per alert on top of global configuration:

This could use exactly the same parameters but within alert definition:

```
alerts: [
{
…
"critical_handlers": ["log", "smtp"],
"warning_handlers": ["log", "smtp"],
"normal_handlers": ["log", "smtp"],
…
}
]
```

Thanks!

No files or tests are named. Start by locating the global alert-handler configuration and the code that parses alert definitions; determine how handler precedence should work when handlers are specified per alert. Done means alert definitions can specify critical, warning, and normal handlers independently while existing global configuration continues to work.

Written by the indexing model from the issue text.