feat: add ExecutionInfo and related types

ExecutionInfo holds data to inform observers of the outcome of an event
handler function's execution. It models the successful and unsuccessful
outcomes of handling an event, which allows ucam_faas to communicate
success/failure via HTTP response, to logs and to automated tests in a
consistent way.

It's extensible, so that function implementers can return their own
subtypes to describe the specific results of their function and get the
general outcome details that apply to any function automatically.