tfexec: Add provided reattach support to all terraform workspace subcommands

[austinvalle]

Related Issue

N/A

Description

This PR adds provider reattach support for all of the terraform workspace subcommands.

The main user of reattach configuration for this Go module is terraform-plugin-testing, which we'll eventually look to expand to support testing pluggable state storage. It's likely we'll look to the terraform workspace subcommands for help writing acceptance tests, so this PR gets ahead of that support.

The only subcommand I think we likely don't need in terraform-plugin-testing is terraform workspace show, but I just added the support for consistency and because I was in the neighborhood 😆 . If we'd prefer I undo that until we have a real use-case, just LMK and I'll update the PR.

I see #555 is WIP, so I just updated the changelog as if it's being prepped for release, although there isn't any urgency ATM 😃

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

No