Add custom headers and base address customization

[timabdulla]

Hi,

This is a simple PR to add the ability to customize the base address (using options[:credentials][:base_address]) and to provide custom headers (using options[:headers]). This is quite useful for integrating with proxy services.

Thanks,

Tim

[JerakRus]

Hi, thanks for the work you've done! I tested your functionality, and it works great!

However, in my case, I used my own proxy server on a VPS, and since my server is just a proxy without a domain (only an IP), I needed more flexibility in configuring SSL certificates for Faraday.

I forked your fork and improved the handling of parameters in connection: { ssl: {...} }. These changes wouldn't make sense without your request, so I could submit a PR to your fork if you're interested, or directly here after your branch is approved.

My fork can be viewed here: JerakRus@9a5a952

[stevenou]

Curious what's happening to this PR or related functionality? I could use custom headers in order to enable control over provisioned throughput usage

[stevenou]

@gbaptista any thoughts on this PR?

[kapso]

@gbaptista Google is increasingly recommending Priority Pay Go Headers to get around 429 errors. Can we merge this PR?

https://docs.cloud.google.com/vertex-ai/generative-ai/docs/priority-paygo#initialize-use-priority