Binance request signing for Insomnia

All private Binance API calls require adding a timestamp and then signing the request (HMAC-SHA256) prior to submitting. Insomnia doesn't have (as far as I can find) a built-in way to do this.

This plugin (when installed) checks all outgoing requests to see if:

• The request has valid context and URL
• The request is going to https://api.binance.com
• The request has a timestamp parameter already
• An Insomnia environment variable api_secret exists
• The request does not already have a signature parameter

If the above conditions are met, this plugin:

• Adds a recvWindow parameter per Binance recommendation. The default value is 5000msec (again, per official recommendations). A future version may allow this to be an environment variable instead.
• Computes the HMAC signature based on the query string of the outgoing request and converts to hex.
• Appends the signature parameter with the computed digest.