In order to a) do it inline with raising, and b) do it pre 3.11 where
Exception.add_note() does not exist.

It flattens nested JSON trees into a single wide object.

This function provides context managers to push values to a ContextVar
within the body of a with-statement and automatically pop the value when
it ends.

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.

It has a type definition for CloudEvent attributes, to make creating
test data more ergonomic.

The handler executor functions are the base level handlers to layer
higher-level, (more specific) handlers on top of. This module's handlers
support reporting function results from ExecutionInfo objects. The
results are made available in 3 ways:

- As HTTP responses to the HTTP request that triggered the event handler
- As structured (JSON) log records
- As the original ExecutionInfo value for unit tests (tests need not
  mock or use a full functions_framework server to run a function to
  test it)

Like .execute(), this allows handler functions to be called directly
with an input value, but execute_for_result() also performs the timing
and logging that happens when handling real events.

This allows handlers to be called for real, without converting the
result into an HTTP response for functions_framework.

High-level handlers MR1: Handler Executors

Closes #31

See merge request !49