aws lambda update-function-configuration: Add --env-file option

often I have this boilerplate code in a shell script:
```sh
source .env

aws lambda update-function-configuration \
--region "$REGION" \
--function-name "$FUNCTION_NAME" \
--environment Variables="{\
VAR1=$VAR1,\
VAR2=$VAR2,\
VAR3=$VAR3,\
VAR4=$VAR4,\
(...)
}" > /dev/null
```

It would be nice if I could do this instead:
```sh
aws lambda update-function-configuration \
--region "$REGION" \
--function-name "$FUNCTION_NAME" \
--env-file .env
```

I'm not a python dev but if you are open to contributions and consider that this feature request makes sense I can propose a PR.