Add public API method for customising captured exceptions

[axw]

Is your feature request related to a problem? Please describe.

Some exceptions may include additional information that is not part of the exception's message or type. Users currently have no means of capturing this information.

Describe the solution you'd like

The exception schema supports recording an error code (string or number), as well as arbitrary key/value attributes. We should provide an API for users to set these for captured exceptions.

Ideally users could register a callback that will be invoked for exceptions of a given type or type-hierarchy. The callback would have the opportunity to set an error code and/or attributes. For reference, the Go agent provides two top-level functions:

• https://godoc.org/go.elastic.co/apm#RegisterErrorDetailer
• https://godoc.org/go.elastic.co/apm#RegisterTypeErrorDetailer

Describe alternatives you've considered

The agent could also (or alternatively) provide an API for capturing an exception without immediately sending it; it would return an object that could be customised, and a have a method for sending it when done.