Add lambda trigger post user signup through Cognito

[mkotsollaris]

Problem:

Currently, creating a new user and a new workspace is done in a different HTTP call. Cognito is responsible for signing in the user and the FN-API POST /workspace is responsible for setting a workspace for a user.

This is problematic since there might be a few edge cases where one operation fails and the other succeeds.

Proposed Solution:

Do a POST to a workspace via a new AWS lambda, creating the workspace for a user. If something goes wrong in either of the reference: https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-post-confirmation.html