Add `coder_file` resource for writing arbitrary files to workspaces

[matifali]

Description

As discussed in coder/coder#3055, there's a need for a dedicated Terraform resource to write arbitrary files into Coder workspaces with proper permission control.

Problem

Currently, template authors need to use workarounds like:

• Environment variables (anti-pattern for large content like service account keys)
• Startup scripts with complex escaping (error-prone)
• Base64 encoding in startup scripts (as shown in the original issue)

Proposed Solution

Introduce a coder_file resource that allows template authors to:

• Write files with specified content to workspace paths
• Set file permissions (mode)
• Handle file ownership appropriately

Example Usage

resource "coder_file" "gcp_credentials" {
  agent_id = coder_agent.dev.id
  path     = "~/.gcp_creds.json"
  content  = google_service_account_key.mykey.private_key
  mode     = "0600"
}

Use Cases

• Service account keys and credentials
• Configuration files, i.e., mcp servers
• Any file content that needs to be dynamically generated

References

• Original request: inject file from coder_agent resource coder#3055
• Used extensively in Coder modules: github.com/coder/registry