feat: Add `disable_module_cache` attribute to `coderd_template` resource

## Summary

The `coderd_template` resource does not currently expose the `disable_module_cache` attribute, which was added to the Coder API in coder/coder#21931 (v2.31+). This means templates managed via Terraform have no way to control this setting.

## Context

Since v2.30 (coder/coder#21398), Terraform modules are cached at template import time and reused for all subsequent workspace builds. A per-template toggle `disable_module_cache` was added in coder/coder#21931 to allow opting out of this behavior.

The setting is available via:
- The Coder UI (Template Settings)
- The REST API (`PATCH /api/v2/templates/{id}` with `disable_module_cache: true`)

But it is **not** available via the `coderd_template` Terraform resource, which is the primary way many teams manage their templates as code.

## Related

- coder/coder#22834 — Tracks the broader gaps: deployment-level flag, TF provider support, v2.30 backport
- coder/coder#22531 — Related bug where module caching breaks relative-path file inclusion

## API Reference

The Coder SDK already supports this field:
- `codersdk.UpdateTemplateMeta.DisableModuleCache` (`*bool`, JSON: `disable_module_cache`)
- Endpoint: `PATCH /api/v2/templates/{id}`

## Proposed Change

Add a `disable_module_cache` boolean attribute to the `TemplateResourceModel` struct in `internal/provider/template_resource.go`, following the same pattern as the recently added `cors_behavior` (#293) and `use_classic_parameter_flow` attributes.

Default should be `false` (caching enabled) to match the server-side default.

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Filed on behalf of @bjornrobertsson