PB-1439: External IDs

[msom]

CC @boecklic

This PR unifies the "external ID" (ID's which are used in the API and/or Cognito).

As per discussion in the ticket, the following changes have been made:

• rename slug to <modelname>_id for Provider, Attribution, Dataset and PackageDistribution
• add slug-like restrictions (a-z0-9_-.) to username`
• add unique constraint to external ID for Dataset and PackageDistribution
• use the external IDs in the API

No changes have been made for User: username is still used in API and user_id for Cognito.

The following table shows an overview of the final state:

Model	External ID	API	Cognito
Provider	provider_id	provider_id as id
Attribution	attribution_id	attribution_id as id
Dataset	dataset_id	dataset_id as id
PackageDistribution	package_distribution_id
User	user_id	username as username	user_id

Checklist

• Model changes documented in the business entity model

[schtibe]

Very nice! 🚀