Add --detached option for sam local start-api

[royassis]

Hey

When testing sam api applications I run the sam local start-api in the background.
Right now I wrote a script that :

1. catches the $pid of the command
2. Run some tests via pytest
3. calls kill -2 $pid

Would be nice if we could have a --detached option for sam local start-api + an option to kill it.