Refactor Unbound setup and add E2E tests (#1340)

Refactor Unbound DNS installation to use modern `conf.d` pattern and add
E2E testing.

**Changes:**
- Unified Unbound config across all distros using
`/etc/unbound/unbound.conf.d/openvpn.conf`
- Added startup validation with retry logic
- Added `ip-freebind` to allow binding before tun interface exists
- E2E tests now verify Unbound DNS resolution from VPN clients

**Testing:**
- Server: verifies config creation, interface binding, security options
- Client: verifies DNS resolution through Unbound (10.8.0.1)

---

Closes https://github.com/angristan/openvpn-install/issues/602 Closes
https://github.com/angristan/openvpn-install/pull/604 Closes
https://github.com/angristan/openvpn-install/issues/1189

Co-authored-by: Henry N <henrynmail-github@yahoo.de>

test/Dockerfile.client

@@ -5,11 +5,13 @@ FROM ubuntu:24.04
 ENV DEBIAN_FRONTEND=noninteractive
 
 # Install OpenVPN client and testing tools
+# dnsutils provides dig for DNS testing with Unbound
 RUN apt-get update && apt-get install -y --no-install-recommends \
     openvpn \
     iproute2 \
     iputils-ping \
     procps \
+    dnsutils \
     && rm -rf /var/lib/apt/lists/*
 
 # Create TUN device directory (device will be mounted at runtime)