feat: add template for externally managed workspaces to coder-labs (#343)

## Description

Add externally-managed-workspace template for connecting Coder
workspaces to externally provisioned compute resources

## Type of Change

- [ ] New module
- [ ] Bug fix
- [x] Feature/enhancement
- [ ] Documentation
- [ ] Other

## Testing & Validation

- [x] Tests pass (`bun test`)
- [x] Code formatted (`bun run fmt`)
- [x] Changes tested locally

## Related Issues

https://github.com/coder/coder/issues/19091

.icons/electric-plug-emoji.svg

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+<svg width="128" height="128" xmlns="http://www.w3.org/2000/svg"><text x="50%" y="50%" font-size="96px" text-anchor="middle" dominant-baseline="middle" font-family="Apple Color Emoji, Segoe UI Emoji, Noto Color Emoji, sans-serif">🔌</text></svg>

registry/coder-labs/templates/externally-managed-workspace/README.md

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+---
+display_name: Externally Managed Workspace
+description: A template to provision externally managed resources as Coder workspaces
+icon: ../../../../.icons/electric-plug-emoji.svg
+verified: true
+tags: [external]
+---
+
+# Externally Managed Workspace Template
+
+This template provides a minimal scaffolding for creating Coder workspaces that connect to externally provisioned compute resources.
+
+Use this template as a starting point to build your own custom templates for scenarios where you need to connect to existing infrastructure.

registry/coder-labs/templates/externally-managed-workspace/main.tf

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+terraform {
+  required_providers {
+    coder = {
+      source  = "coder/coder"
+      version = ">= 2.10"
+    }
+  }
+}
+
+data "coder_parameter" "agent_config" {
+  name         = "agent_config"
+  display_name = "Agent Configuration"
+  description  = "Select the operating system and architecture combination for the agent"
+  type         = "string"
+  default      = "linux-amd64"
+
+  option {
+    name  = "Linux AMD64"
+    value = "linux-amd64"
+  }
+  option {
+    name  = "Linux ARM64"
+    value = "linux-arm64"
+  }
+  option {
+    name  = "Linux ARMv7"
+    value = "linux-armv7"
+  }
+  option {
+    name  = "Windows AMD64"
+    value = "windows-amd64"
+  }
+  option {
+    name  = "Windows ARM64"
+    value = "windows-arm64"
+  }
+  option {
+    name  = "macOS AMD64"
+    value = "darwin-amd64"
+  }
+  option {
+    name  = "macOS ARM64 (Apple Silicon)"
+    value = "darwin-arm64"
+  }
+}
+
+data "coder_workspace" "me" {}
+
+locals {
+  agent_config = split("-", data.coder_parameter.agent_config.value)
+  agent_os     = local.agent_config[0]
+  agent_arch   = local.agent_config[1]
+}
+
+resource "coder_agent" "main" {
+  arch = local.agent_arch
+  os   = local.agent_os
+}
+
+resource "coder_external_agent" "main" {
+  agent_id = coder_agent.main.id
+}
+
+# Adds code-server
+# See all available modules at https://registry.coder.com/modules
+module "code-server" {
+  count  = data.coder_workspace.me.start_count
+  source = "registry.coder.com/coder/code-server/coder"
+
+  # This ensures that the latest non-breaking version of the module gets downloaded, you can also pin the module version to prevent breaking changes in production.
+  version = "~> 1.0"
+
+  agent_id = coder_agent.main.id
+}