Considere addinng postinstall script for installing

**Actual Behaviour**

Nothing happens

**Expected Behaviour**

I wanted the plugin to execute `serverless dynamodb install` as a post-install script

Hello, thanks for this great plugin.
Any reason why you don't make the installation automatic as a post-install script?
I always forget about it and having it happen automatic will be fantastic.
Regards

Start with the `serverless dynamodb install` command and the plugin's package installation behavior. Determine where a postinstall hook would be configured and how the automatic installation can be verified. Done means installing the plugin triggers the requested local DynamoDB installation without requiring a separate command.

Written by the indexing model from the issue text.