Websocket proxy with string replacing

• Configured with environment variables, at least BACKEND needs to be set with a ws:// or wss:// URL
• This application is a Websocket server providing an websocket endpoint listening on PORT (8080 by default)
• the client request path is ignored, so /foobar will also get proxied to the same BACKEND URL
• GET request parameters from the client (e.g. /foobar?x=y) are appended to the BACKEND URL?x=y
• all websocket messages/queries from clientt are forwarded to BACKEND and vice versa
• all messages from BACKEND will be forwarded to the client
• to use ssl-client-certificates with BACKEND put them in SSL_CLIENT_CERT/SSL_CLIENT_KEY env variables
• Verification for the CA-certificate in SSL_CLIENT_CA is implemented but untested
• (all certs/keys in PEM format starting with "-----BEGIN..." includinig newlines)
• on the answer of BACKEND string replacement can be performed: all occurrences of OLD_HOST strings are replaced with NEW_HOST (e.g. with OLD_HOST=foo and NEW_HOST=bar the BACKEND message '{"message": "Hello World", "url": "https://foo.com/"}' would then be forwarded to the client as: '{"message": "Hello World", "url": "https://bar.com/"}'