Feature: install self-signed cacert to trust store

For analysis and reverse engineering, it can be helpful to insert a
custom CA certificate into Waydroid’s system-wide trust store.
Users used to be able to do that via Android’s settings but not anymore.

The `install mitm` command accepts a path to a file that contains a
– typically self-signed – CA certificate in PEM format.
It then renames [1] and copies the file into the overlay file system,
placing it into Waydroid’s trust store.

As a usage example, the following command lines enable your host to use
mitmproxy [2] to act as a proxy and to intercept [3] HTTP(S) connections
that come from the Waydroid container:

```sh
$ timeout --preserve-status 2 mitmdump -n                           # creates a CA cert in ~/.mitmproxy
$ sudo venv/bin/python3 main.py install mitm --ca-cert ~/.mitmproxy/mitmproxy-ca-cert.pem
INFO: Creating directory: /system/etc/security/cacerts
INFO: Copying /home/yourname/.mitmproxy/mitmproxy-ca-cert.pem to system trust store
INFO: Target file: /system/etc/security/cacerts/6320a7db.0
INFO: mitm installation finished
$ sudo waydroid shell -- ls -l /system/etc/security/cacerts         # double-check that it worked
[…]
-rw-r--r-- 1 root root 1191 2024-01-01 00:00 6320a7db.0
[…]
$ adb shell settings put global http_proxy ${YOUR_IP_HERE?}:3128    # tell Waydroid to use the proxy
                                                                    #     for all connections
$ mitmproxy -p 3128                                                 # start proxy and display a TUI
                                                                    #     with HTTP(S) connections
                                                                    #     coming from Waydroid
```

[1]: https://docs.mitmproxy.org/stable/howto-install-system-trusted-ca-android/#2-rename-certificate

[2]: https://mitmproxy.org/

[3]: https://docs.mitmproxy.org/stable/mitmproxytutorial-interceptrequests/

README.md

@@ -29,7 +29,7 @@ cd waydroid_script
 python3 -m venv venv
 venv/bin/pip install -r requirements.txt
 # install something
-sudo venv/bin/python3 main.py install {gapps, magisk, libndk, libhoudini, nodataperm, smartdock, microg}
+sudo venv/bin/python3 main.py install {gapps, magisk, libndk, libhoudini, nodataperm, smartdock, microg, mitm}
 # uninstall something
 sudo venv/bin/python3 main.py uninstall {gapps, magisk, libndk, libhoudini, nodataperm, smartdock, microg}
 # get Android device ID
@@ -122,6 +122,12 @@ Open terminal and switch to directory where "main.py" is located then run:
 
     sudo venv/bin/python3 main.py install smartdock
 
+## Install a self-signed CA certificate
+
+Open terminal and switch to directory where "main.py" is located then run:
+
+    sudo venv/bin/python3 main.py install mitm --ca-cert mycert.pem
+
 ## Granting full permission for apps data (HACK)