Add logging of dashboard create and update payloads

[cory-stripe]

Summary

Adds logging of dashboard create and update payloads.

Motivation

When debugging new features I keep hacking in weird ways to view the payload. This — when used with TF_LOG — allows easy inspection of the body!

Notes

I'm following convention used in the Datadog provider. It seems this should really be a log level, but log levels in terraform seem broken anyway.

Let me know if you'd like to see this in a different shape!