Implement the proxy in main in the runtimes

[sciabarracom]

The runtime proxy (repo: apache/openserverless-runtime) will support proxy with a main in this format:

<main>@<proxy>

so you provide for example --main=main@vastai.nuvolaris.io the runtime will behave as follow:

• at init, instead of spawning a new executor will simply make a connection to vastai.nuvolaris.io and execute the init posting the payload
• at run, instead of sending requests to the executor will send them to the remote one.

Some notes on the sintax:

• @<proxy> allows for the default main, so if you provide --main=@<proxy> it shold forward the request as no main was provide using the defailt for the runtimes
• if the <proxy> is in the format of an internet domain xxx.yyy.zzz the forwarding should happen with https: protocol, howerver if you use http://xxx.yyy.zzz the forwarding should happen using the http protocol

To awoid security issues, enable the beaviour only if the the variable OW_USE_PROXY_MAIN is provided, so we will build a proxy image (that will be just the proxy without anything else)

[francescotimperi]

To add a bit more of context the idea it is to create a runtime with proxy capabilities which will assume that

• an env variable OW_USE_PROXY_MAIN=1
• action will be created specifying a --main main@<hostname>:<port>

Under this circumstances, the proxy will have to forward the request to a runtime running at the https://<hostname>:<port>, but the request has to be rewritten in a way that the --main params is transformed into something like --main main. The format main@<hostname>:<port> it not understood by any of the OpenWhisk action loop based runtimes, as they are only able to parse something like --main main or --main class#method (for Java).