Add schema validation for custom serverless configuration

[robin-thomas]

We are using the custom property (from serverless.yml) here:

serverless-aws-secrets/src/index.ts

Line 64 in 5ff4bb3

this.options = serverless.service.custom?.['serverless-aws-secrets'] ?? {};

Add schema validation for these properties, like mentioned here: https://www.serverless.com/framework/docs/guides/plugins/custom-configuration