feat(cbindings): tiny waku relay example in Python (#1793)

* Adding an example on how to integrate nwaku library (libwaku.so) in Python
* Adding requirements file

examples/python/requirements.txt

@@ -0,0 +1,7 @@
+blinker==1.6.2
+click==8.1.6
+Flask==2.3.2
+itsdangerous==2.1.2
+Jinja2==3.1.2
+MarkupSafe==2.1.3
+Werkzeug==2.3.6

examples/python/waku.py

@@ -0,0 +1,117 @@
+from flask import Flask
+import ctypes
+import argparse
+
+libwaku = object
+try:
+    # This python script should be run from the root repo folder
+    libwaku = ctypes.CDLL("build/libwaku.so")
+except Exception as e:
+    print("Exception: ", e)
+    print("""
+The 'libwaku.so' library can be created with the next command from
+the repo's root folder: `make libwaku`.
+
+And it should build the library in 'build/libwaku.so'.
+
+Therefore, make sure the LD_LIBRARY_PATH env var points at the location that
+contains the 'libwaku.so' library.
+""")
+    exit(-1)
+
+def handle_event(event):
+    print("Event received: {}".format(event))
+
+def call_waku(func):
+    ret = func()
+    if (ret != 0):
+        print("Error in %s. Error code: %d" % (locals().keys(), ret))
+        exit(1)
+
+# Parse params
+parser = argparse.ArgumentParser(description='libwaku integration in Python.')
+parser.add_argument('-d', '--host', dest='host', default='0.0.0.0',
+                    help='Address this node will listen to. [=0.0.0.0]')
+parser.add_argument('-p', '--port', dest='port', default=60000, required=True,
+                    help='Port this node will listen to. [=60000]')
+parser.add_argument('-k', '--key', dest='key', default="", required=True,
+                    help="""P2P node private key as 64 char hex string.
+e.g.: 364d111d729a6eb6d2e6113e163f017b5ef03a6f94c9b5b7bb1bb36fa5cb07a9""")
+parser.add_argument('-r', '--relay', dest='relay', default="true",
+                    help="Enable relay protocol: true|false [=true]")
+parser.add_argument('--peer', dest='peer', default="",
+                    help="Multiqualified libp2p address")
+
+args = parser.parse_args()
+
+# The next 'json_config' is the item passed to the 'waku_new'.
+json_config = "{ \
+                \"host\": \"%s\",   \
+                \"port\": %d,       \
+                \"key\": \"%s\",    \
+                \"relay\": %s      \
+            }" % (args.host,
+                  int(args.port),
+                  args.key,
+                  "true" if args.relay else "false")
+
+callback_type = ctypes.CFUNCTYPE(None, ctypes.c_char_p, ctypes.c_size_t)
+
+# Retrieve the current version of the library
+libwaku.waku_version(callback_type(lambda msg, len:
+                                  print("Git Version: %s" %
+                                        msg.decode('utf-8'))))
+# Retrieve the default pubsub topic
+default_pubsub_topic = ""
+libwaku.waku_default_pubsub_topic(callback_type(
+    lambda msg, len: (
+        globals().update(default_pubsub_topic = msg.decode('utf-8')),
+        print("Default pubsub topic: %s" % msg.decode('utf-8')))
+))
+
+print("Bind addr: {}:{}".format(args.host, args.port))
+print("Waku Relay enabled: {}".format(args.relay))
+
+# Node creation
+libwaku.waku_new.argtypes = [ctypes.c_char_p,
+                             callback_type]
+
+libwaku.waku_new(bytes(json_config, 'utf-8'),
+                       callback_type(
+                           #onErrCb
+                           lambda msg, len:
+                           print("Error calling waku_new: %s",
+                                 msg.decode('utf-8'))
+                           ))
+# Start the node
+libwaku.waku_start()
+
+# Set the event callback
+callback_type = ctypes.CFUNCTYPE(None, ctypes.c_char_p)
+callback = callback_type(handle_event)
+libwaku.waku_set_event_callback(callback)
+
+# Subscribe to the default pubsub topic
+libwaku.waku_relay_subscribe(default_pubsub_topic.encode('utf-8'),
+                                    callback_type(
+                                        #onErrCb
+                                        lambda msg, len:
+                                        print("Error calling waku_new: %s",
+                                                msg.decode('utf-8'))
+                                        ))
+
+libwaku.waku_connect(args.peer.encode('utf-8'),
+                     10000,
+                     callback_type(
+                     # onErrCb
+                     lambda msg, len:
+                     print("Error calling waku_new: %s", msg.decode('utf-8'))))
+
+# app = Flask(__name__)
+# @app.route("/")
+# def hello_world():
+#     return "Hello, World!"
+
+# Simply avoid the app to
+a = input()
+