Allow customizing the Axios instance in the JS client

Hi,

In some cases the admin section of Ghost can be used behind a proxy that requires specific HTTP elements (headers for cookie or anything else).

Right now the Axios HTTP client used is not customizable https://github.com/TryGhost/SDK/blob/main/packages/content-api/lib/index.js so the requests cannot be custom. I think it would be great to be able passing an instance either at init or at each request, if none, just use the default Axios import.

What do you think?

Thank you,

PRs are welcome :wink: If you look at the admin-api package then you can see how makeRequest can be passed in as an option, the same pattern could be followed in content-api.