Feature: Run `sls dynamodb start` in detached mode

For CI testing, one would need to run `sls dynamodb start` asynchronously, because the command blocks the current shell.

**Workaround 1**: `sls dynamodb start &`
This runs it in the background, but does not wait until the db is set up properly.

**Workaround 2**: `sls dynamodb start & sleep 5`
This works, but might introduce race conditions.

Suggestion:
How about: `sls dynamodb start --detached` which runs synchronously until all work is done, exits the process (the shell command is finished), but continues serving the database in the background.

Start at the implementation and tests for the `sls dynamodb start` command, then trace how the local database process is started and how the command currently blocks. Define the detached-mode lifecycle from the issue: wait for setup, exit the shell command, and keep the database serving in the background. Add coverage for the new option and its readiness behavior.

Written by the indexing model from the issue text.