Replace flask development server backend with production one

[t-ambur]

We should stop using flask server and replace it with a production one. Since we are using SocketIO, one of the highest recommended packages is 'eventlet'.

Add 'eventlet' to 'install_requires' in setup.py as a required dependency.

Then update some of the code for socketio in graphex/server.py:

socketio = SocketIO(
    app,
    async_mode="eventlet",            # make the backend explicit
    path="/api/socket.io",
    cors_allowed_origins="*",
    logger=False,
    engineio_logger=False
)

and

self.socketio.run(
    app=self.app,
    host="0.0.0.0",
    port=port,
    ssl_context=self.ssl_context,
    debug=False
)

Then do some testing to determine nothing is broken with executing existing graphs.

[t-ambur]

In further reading about eventlet, it is not recommended for usage anymore:
"New usages of eventlet are now heavily discouraged!"
from https://pypi.org/project/eventlet/

We should review other options for getting off of the flask development server.

Edit:
According to the docs for Flask-SocketIO these are the supported options:
eventlet, gevent and the Flask development server.
(https://flask-socketio.readthedocs.io/en/latest/deployment.html)
(also see https://flask-socketio.readthedocs.io/en/latest/api.html#flask_socketio.SocketIO)

It seems as though gevent should be the next tested replacement for the threaded async mode we are currently using of Flask development server. This module still appears to be maintained: https://pypi.org/project/gevent/#description

It still requires a monkey_patch. We may need to be a bit more surgical with our import statements and test more thoroughly. I recommend a major version update for this issue.

Possible needed new pip modules:

• gevent
• gevent-websocket

Code idea snippets:
In server.py:

        app = flask.Flask(__name__, static_url_path="", static_folder=website_path)
        self.app = app

        logging.getLogger("werkzeug").setLevel(logging.WARNING)
        socketio_logger = logging.getLogger("socketio")
        engineio_logger = logging.getLogger("engineio")
        socketio_logger.setLevel(logging.WARNING)
        engineio_logger.setLevel(logging.WARNING)

        # Use gevent async mode for WebSockets
        self.socketio = SocketIO(
            app,
            async_mode="gevent",
            path="/api/socket.io",
            cors_allowed_origins="*",
            logger=socketio_logger,
            engineio_logger=engineio_logger
        )

In server.py:

    def start(self, port: int = 80):
        print(f"GraphEx server starting on all network interfaces at port: {port}")
        cert_path, key_path = self.ssl_context
        # socketio.run will use gevent when installed
        self.socketio.run(
            app=self.app,
            host="0.0.0.0",
            port=port,
            certfile=cert_path,
            keyfile=key_path,
            debug=self.flask_debug_logging
        )

In main.py:

# Apply gevent patching only for 'serve' mode and as early as possible
if MODE == "serve":
    from gevent import monkey
    monkey.patch_all()

# Now import third-party and graphex modules
from getpass import getpass
from cryptography.fernet import InvalidToken
from graphex import (
    Graph,
    GraphConfig,
    GraphexLogger,
    GraphRegistry,
    GraphRuntime,
    GraphServer,
    vault,
    GraphInputValueMetadata,
    GraphInventory
)