Refactor Context API: Credentials + Client Architecture

[simon-contreras-deel]

in this doc we decided to refactor the internal architecture to support:

• Different environments of execution (local/APIClient and analysis/PGClient)
• Allow OAuth integration, with the token refresh mechanism

So basically these are the tasks to achieve the refactor:

• Create client classes Client Architecture #833
  • BaseClient
  • APIClient: contains sql_client, copy_client, batch_client
• Refactor Dataset to instantiate and use a client Decoupling clients from Dataset #831
• New credentials
  • Refactor Credentials (username, api_key, base_url)
  • using new credentials
• Usingn Credentials instead of Context
  • Use Credentials instead of Context in Dataset
  • Use Credentials instead of Context in Source
  • Use Credentials instead of Context in Layer
  • Use Credentials instead of Context in Map
  • Use Credentials instead of Context in Kuviz classes
  • Use Credentials instead of Context in clients

[simon-contreras-deel]

Depends on #831

[simon-contreras-deel]

From this comment #832 (comment)

And a naming suggestion: I know we were using creds beforehand, but from now on I'd prefer not abbreviating public API variable names, even more when they have a related class.