Ability to specify custom rules to set `event.outcome`

**Is your feature request related to a problem? Please describe.**
A transaction representing a service invocation success or failure can't always be defined by a return status. Sometimes, HTTP response code 2xx is not a sufficient indication of success and there is a need to explore additional attributes to be able to confidently set `event.outcome` correctly. The same applies for designated a transaction as a failure.

**Describe the solution you'd like**
As a service developer, I want to be able to define more complex rules to set `event.outcome` to success or failure based on the additional information either available as transaction attributes, or through an API.

**Describe alternatives you've considered**
Using public APIs is possible, but negates the value of autodetection completely.

**Additional context**
Without this ability, the error rate indication in the APM UI is not showing the correct picture of failures.