Update entrypoint.sh :: Convert envs to lowercase & remove `input_`

[ranma2913]

I'd like to suggest the following features:

• Iterate over environment variables starting with INPUT_ and export them as lowercase variables without input_ prefix.
  • I don't think you need the elif [ -n "$MAX_TIME" ]; then; max_time=$MAX_TIME; fi blocks because users should only use the inputs from a with directive as described in all the readme.
• Add a trap config to print the line number of any errors. This was immensely useful for my team to debug and understand how to use your action.
• Add Connection: close header to curl requests.
  • Helps the CURL requests exit faster as well as letting the targets release their connection faster. Too many /github-webhook/ calls on Jenkins instances cause latency from my experience.
• Update User-Agent: GitHub-Hookshot/$REQUEST_ID to be dynamic.
  • The Delivery headers docs don't really explain how this header is used but should it be hard-coded to some value we don't know what it means?
• Change to build headers & auth_header just like the curl_options. This way we can have just 1 place we actually make a curl request.
• Add curl_connection_close input to optionally disable adding of the -H Connection: close header.