Add `coder_file` resource for writing arbitrary files to workspaces

## 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

```hcl
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: coder/coder#3055
- Used extensively in Coder modules: github.com/coder/registry