Feature Request: Disable lambda_unhandled_exception_warning_message

The lambda_unhandled_exception_warning_message (`LAMBDA_WARNING: Unhandled exception...`) is ~ 633 chars and adds little value to our debugging. I understand the desire to warn people the runtime may have changed underfoot, and that's why their code is raising exceptions, but I'd like a way to disable it.

A pr from me would probably be something like

```
_disable_unhandled_exception_warning = bool(os.environ.get("AWS_LAMBDA_EXCEPTION_WARNING_DISABLE"))
# SNIP ...
if _disable_unhandled_exception_warning:
from .lambda_literals import lambda_unhandled_exception_warning_message

log_sink.log(lambda_unhandled_exception_warning_message, _WARNING_FRAME_TYPE)

log_error(error_result, log_sink)
lambda_runtime_client.post_invocation_error(
invoke_id, to_json(error_result), to_json(xray_fault)
)
```
near https://github.com/aws/aws-lambda-python-runtime-interface-client/blob/056aa365b6b986b0f413d031f054598064df603e/awslambdaric/bootstrap.py#L217

Perhaps you have a different idea? Or maybe this is a non-starter. Didn't want to clutter PRs before asking.