feat: add support for authorization header

[praetoriansentry]

Description

Adding a flag to add an authorization bearer to http requests.

Tests

Ensure polycli monitor is working:

./out/polycli monitor -H "X-goog-api-key: $apikey" --rpc-url https://json-rpc.do9ic5iuox53t826lv7lqek2p.blockchainnodeengine.com

Ensure loadtest command works with the custom header as well:

./out/polycli loadtest --mode rpc -H "X-goog-api-key: $apikey" --rpc-url https://json-rpc.do9ic5iuox53t826lv7lqek2p.blockchainnodeengine.com --verbosity 700 --requests 100 --concurrency 10 --rate-limit 50