Feature Request: Performant external data

[bmulholland]

Is your feature request related to a problem? Please describe.

When using Nylas data without external data, each message is loaded on demand. But when using external data, the messages must all be loaded up front.

Describe the solution you'd like

A way to load message content on demand to reduce up-front loading times.

Describe alternatives you've considered

Loading all messages up front works, but requires fetching every single message that may be displayed, even though they're unlikely to.