XMLStream: allow custom sslcontext provisioning (fixes #3582)

For some applications that have strict requirements on blocking calls,
it might be beneficial to create the SSLContext in advance and
provide it to the client/componentxmpp instance that will be going
through kwargs until XMLStream.

The context will be reconfigured later on based on user parameters, but
it is highly recommended to set it up in a secure way.

slixmpp/xmlstream/xmlstream.py

@@ -281,7 +281,8 @@ class XMLStream(asyncio.BaseProtocol):
     __slow_tasks: List[Task]
     __queued_stanzas: List[Tuple[Union[StanzaBase, str], bool]]
 
-    def __init__(self, host: str = '', port: int = 0):
+    def __init__(self, host: str = '', port: int = 0,
+                 ssl_context: Optional[ssl.SSLContext] = None):
         self.transport = None
         self.socket = None
         self._connect_loop_wait = 0
@@ -298,9 +299,12 @@ class XMLStream(asyncio.BaseProtocol):
         # A dict of {name: handle}
         self.scheduled_events = {}
 
+        if ssl_context is None:
             self.ssl_context = ssl.create_default_context()
             self.ssl_context.check_hostname = True
             self.ssl_context.verify_mode = ssl.CERT_REQUIRED
+        else:
+            self.ssl_context = ssl_context
 
         self.event_when_connected = "connected"