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/pia/portforward.go

@@ -0,0 +1,46 @@
+package pia
+
+import (
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+
+	"github.com/qdm12/private-internet-access-docker/internal/constants"
+	"github.com/qdm12/private-internet-access-docker/internal/models"
+)
+
+func (c *configurator) GetPortForward() (port uint16, err error) {
+	c.logger.Info("%s: Obtaining port to be forwarded", logPrefix)
+	b, err := c.random.GenerateRandomBytes(32)
+	if err != nil {
+		return 0, err
+	}
+	clientID := hex.EncodeToString(b)
+	url := fmt.Sprintf("%s/?client_id=%s", constants.PIAPortForwardURL, clientID)
+	content, status, err := c.client.GetContent(url)
+	if err != nil {
+		return 0, err
+	} else if status != 200 {
+		return 0, fmt.Errorf("status is %d for %s; does your PIA server support port forwarding?", status, url)
+	} else if len(content) == 0 {
+		return 0, fmt.Errorf("port forwarding is already activated on this connection, has expired, or you are not connected to a PIA region that supports port forwarding")
+	}
+	body := struct {
+		Port uint16 `json:"port"`
+	}{}
+	if err := json.Unmarshal(content, &body); err != nil {
+		return 0, fmt.Errorf("port forwarding response: %w", err)
+	}
+	c.logger.Info("%s: Port forwarded is %d", logPrefix, port)
+	return body.Port, nil
+}
+
+func (c *configurator) WritePortForward(filepath models.Filepath, port uint16) (err error) {
+	c.logger.Info("%s: Writing forwarded port to %s", logPrefix, filepath)
+	return c.fileManager.WriteLinesToFile(string(filepath), []string{fmt.Sprintf("%d", port)})
+}
+
+func (c *configurator) AllowPortForwardFirewall(device models.VPNDevice, port uint16) (err error) {
+	c.logger.Info("%s: Allowing forwarded port %d through firewall", logPrefix, port)
+	return c.firewall.AllowInputTrafficOnPort(device, port)
+}