feat(coderd/templatebuilder): add exampleID->OS map and base template .tf.tmpl files (#26115)

Add the bundled `exampleID -> OS` Go map for Docker, Kubernetes, and AWS
EC2 Linux base templates. Create `.tf.tmpl` Go template files for each
within `coderd/templatebuilder/bases/`, along with `BaseRenderContext`
and `RenderBaseTemplate` rendering helpers.

The `.tf.tmpl` files are independent copies of the example templates
with module blocks (code-server, jetbrains) removed, since the template
builder composes modules separately into `modules.tf`. When
`ImageOptions` is provided, the container image field references the
Terraform parameter; otherwise it uses the hardcoded value via Go
template whitespace control (`{{-`).

Golden file snapshot tests verify rendered output stability with an
`-update` flag for regeneration.

Depends on #25909

> [!NOTE]
> This PR was authored by Coder Agents on behalf of @jeremyruppel.

coderd/templatebuilder/bases/aws-linux/cloud-init/cloud-config.yaml.tftpl

@@ -0,0 +1,8 @@
+#cloud-config
+cloud_final_modules:
+  - [scripts-user, always]
+hostname: ${hostname}
+users:
+  - name: ${linux_user}
+    sudo: ALL=(ALL) NOPASSWD:ALL
+    shell: /bin/bash

coderd/templatebuilder/bases/aws-linux/cloud-init/userdata.sh.tftpl

@@ -0,0 +1,2 @@
+#!/bin/bash
+sudo -u '${linux_user}' sh -c '${init_script}'