Support recent TFC/TFE API changes

[alex-ikse]

Use-cases

Each version of TFE/TFC add new API endpoints and/or new attributes, but go-tfe is not aligned automatically.

When we want to add these features to terraform-provider-tfe we are limited by go-tfe support.

Attempted Solutions

No solution witout a go-tfe upgrade

Proposal

I've filled 2 PRs (#837 and #836) to add features I need to go-tfe (to be able to propose a PR for terraform-provider-tfe)

[brandonc]

We've evaluated a few methodologies in the past to code-generate this SDK, but there are a few technical hurdles. Internally, we try and keep go-tfe and the provider aligned with new features as they are released. I have addressed the individual PRs directly and appreciate you reporting and contributing the missing API attributes! And for this reminder to look into them!