Support Terraform import for emqxcloud_deployment (0.2.0)

[zibuyu1995]

Target: 0.2.0. Not for the 0.1.0 release, which documents import as unsupported.

Problem

No resource implements ImportState. Combined with the deployment lifecycle boundary, emqxcloud_deployment becomes a one-way door:

create deployment -> provider refuses delete -> terraform state rm to release
                  -> Terraform can never manage that deployment again

After terraform state rm, a deployment remains readable through the emqxcloud_deployment data source but cannot be managed by any resource. Live deployment identifiers and cleanup ownership are intentionally tracked outside this public issue.

The provider can read a deployment by ID but cannot adopt one. Any lost state file or mistaken state rm therefore requires manual recovery.

Suggested scope

• Add ImportState to emqxcloud_deployment, keyed by deployment_id, and populate immutable attributes through the Platform API read path.
• Define handling for deployment_name and a missing deployment_version.
• Consider import for Deployment API resources whose identities are already well defined (type:name, rule_id, user_id, as/who).
• Update README.md, docs/index.md, and templates/*.tmpl, which currently state that import is unsupported.

This changes externally visible behavior, so it requires a confirmed development document first, per AGENTS.md.