Allow custom headers for client

[rich-fulker]

Is your feature request related to a problem? Please describe.
We have our harbor instance behind a company WAF which requires a custom header to get through. Tried to use the provider from BESTSELLER in the past, but had to fork to update. Would be great if this was supported in this provider

Describe the solution you'd like
Allow the option to pass a custom header to the provider when initialising, either as a parameter or an ENV

Describe alternatives you've considered
forking and maintaining my own version

[flbla]

hi @rich-fulker,
thanks for sharing your use case
could you please create a PR from your fork ?, I'll review it 😊