TFC: Consider support for run tasks

[radeksimko]

Background

As part of #1590 we introduced structured log output support for different parts of run.

Run tasks, which typically run after any built-in actions like "planned_changes" were initially not considered. This is partly because they don't appear to be included in the machine-readable log output we consume to produce the UI elements currently.

Run tasks in the API are found here: https://developer.hashicorp.com/terraform/cloud-docs/api-docs/run-tasks/run-tasks

Proposal

Read up on the API docs and UX implications and decide how/if we could integrate run tasks in the TFC plan/apply panel view.