feat: create, update, and delete integrations

[toph-allen]

Superseded by #440.

Intent

Add functions to create, update, and delete integrations.

Fixes #434

Approach

• Three pretty simple functions to create, update, and delete integrations.
• create_integration() takes a client.
• update_integration() and delete_integration() take a connect_integration object as their first argument.
  • To support this, the connect_integration class now gets a client as an attribute when it is created.

Checklist

• Does this change update NEWS.md (referencing the connected issue if necessary)?
• Does this change need documentation? Have you run devtools::document()?

[toph-allen]

Closing in favor of #440.