Implement corefunc_println

[skyzyx]

Go: corefunc.Println(string)
TF: corefunc_println

Print a custom line of text to the plan/apply output.

• Add color support?
• Would support heredocs, of course.

[skyzyx]

Although I was able to run acceptance tests, and tfschema worked, Terraform would fail when running v1.6.0. Switched to v1.5.7 and Terraform stopped failing.

Unfortunately, this entire feature is currently blocked by these issues (maybe more):

• Filed 2018-02-02 — Allow resources to report more detailed progress from CRUD hashicorp/terraform#17267
• Filed 2021-04-01 — Support option to output stderr regardless of failure hashicorp/terraform-provider-external#67
• Filed 2023-04-03 — feat: Also log command stderr at TRACE level hashicorp/terraform-provider-external#206

[skyzyx]

Completed work (until the issues are resolved) is in https://github.com/northwood-labs/terraform-provider-corefunc/tree/feature/message.