Get all integrations associated with a piece of content

[toph-allen]

In #431, we added the ability to set integrations for a piece of content. It needs a corresponding getter function.

The API (and the Python SDK) return an association object, which contains a subset of the integration data, plus the date created. For connectapi, we're going to implement a get_integrations() function that does some extra legwork to just return the integrations themselves.

GUID <- "123"
client <- connect()

content <- content_item(client, GUID)

# I would expect this to return a list of integrations, like `get_integrations(client)`.
integrations <- get_integrations(content)

We may add a get_oauth_associations function at a later date if it's important.