feat: add handler executor functions

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)