Add an on_start_event hook for connecting databases - stop the authorizer connecting to the database on launch

Using globals and then having if AWS_CHALICE_CLI_MODE to check whether we should connect to our database or not feels hacky. It would be very nice to have an on startup event hook so that we can add code such as connecting to our database / reporting etc. to the lambda start event.

Just a callback like:
```python
def on_server_startup():
startup_code()
```
would be amazing

The issue names no files, tests, or entry points. Start by tracing the launch path around the AWS_CHALICE_CLI_MODE check and the authorizer's database connection, then establish the startup hook's timing and expected behavior before defining completion with tests for callback execution.

Written by the indexing model from the issue text.