tfexec: Add (Terraform).SetLogCore() and (Terraform).SetLogProvider() methods

[bflad]

Reference: hashicorp/terraform-plugin-sdk#992
Closes #101

[bflad]

Looks like the unit test failure is due to upstream etcd backend removal that was just merged in.

=== RUN   TestShow_noInitEtcdBackend/etcd_backend-refs/heads/main
    util_test.go:173: [INFO] running Terraform command: /tmp/hc-install-terraform-acb79a75451e39126f1cdc596ff802a91d64d7123921547818/terraform show -json -no-color
    show_test.go:225: expected error ErrNoInit, got *fmt.wrapError: exit status 1
        
        Error: Invalid backend type
        
          on main.tf line 2, in terraform:
           2:   backend "etcd" {
        
        The "etcd" backend is not supported in Terraform v1.3 or later.
--- FAIL: TestShow_noInitEtcdBackend (0.06s)
    --- FAIL: TestShow_noInitEtcdBackend/etcd_backend-refs/heads/main (0.06s)

I'll try to separately fix that.

[bflad]

Test fix pull request: #325