Prepare for introduction of terraform-provider-pipecd

[kentakozuka]

• Create namespace(pipe-cd)
• Setup CI
• Create a repository
• Preparing and Adding a Signing Key
• Set GPG key and pass phrase to Secrets
• Publishing Your Provider
• Sync on Terraform Registry

ref: https://developer.hashicorp.com/terraform/registry/providers/publishing#publishing-to-the-registry

[t-kikuc]

I think this is closed.
If not, please open again.