Add support for setting a custom http agent

[steelbrain]

This PR adds a new setHttpAgent method to the API that sets the options.agent for requests.

The name is inspired from a similar method in stripe-node but I am open to changing it for a better name

[rlopzc]

Hello @steelbrain

Why do you want to set a custom http agent? Is there an specific use case?

[steelbrain]

@romariolopezc This is required for when a business does not want to store things like service credentials themselves, and has a reverse proxy that strips creds -> own servers -> forward proxy that injects creds like VGS.

While reverse proxy works by pointing client side apps to VGS, forward proxy needs to be an http agent on the requests. Adding this in Regaliator would allow Regalii usage in such setups