Add serverless v3 compatibility

[somq]

Free-form CLI options are deprecated in serverless v3, therefore this plugin does not work anymore.

• Added cliOptions getter with v2 backward compatibility
• Untested with v2! (though it should stillbe working seamlessly)

[somq]

This PR can be referenced directly in package.json in the meanwhile it is being merged.

Example package.json

...
  "dependencies": {
    "fullstack-serverless": "somq/fullstack-serverless#chore/slsv3-compat"
   ...
}