Rewrite of the entrypoint in Golang (#71)

- General improvements
    - Parallel download of only needed files at start
    - Prettier console output with all streams merged (openvpn, unbound, shadowsocks etc.)
    - Simplified Docker final image
    - Faster bootup
- DNS over TLS
    - Finer grain blocking at DNS level: malicious, ads and surveillance
    - Choose your DNS over TLS providers
    - Ability to use multiple DNS over TLS providers for DNS split horizon
    - Environment variables for DNS logging
    - DNS block lists needed are downloaded and built automatically at start, in parallel
- PIA
    - A random region is selected if the REGION parameter is left empty (thanks @rorph for your PR)
    - Routing and iptables adjusted so it can work as a Kubernetes pod sidecar (thanks @rorph for your PR)

internal/constants/vpn.go

@@ -0,0 +1,21 @@
+package constants
+
+import (
+	"github.com/qdm12/private-internet-access-docker/internal/models"
+)
+
+const (
+	// PrivateInternetAccess is a VPN provider
+	PrivateInternetAccess models.VPNProvider = "private internet access"
+	// Mullvad is a VPN provider
+	Mullvad models.VPNProvider = "mullvad"
+	// Windscribe is a VPN provider
+	Windscribe models.VPNProvider = "windscribe"
+)
+
+const (
+	// TCP is a network protocol (reliable and slower than UDP)
+	TCP models.NetworkProtocol = "tcp"
+	// UDP is a network protocol (unreliable and faster than TCP)
+	UDP models.NetworkProtocol = "udp"
+)