Add authentication support for mirroring

[mishamyte]

Is your feature request related to a problem? Please describe.

Now BaGet could mirror only public feeds. Would be nice to have a support of authentication at external feeds with a supported ways.

Use case behind feature request:
At my work we are using microservices approach, where each service has it's own solution. Contracts (integration events etc) are build into packages and pushed to a private NuGet (we are using Nexus). At development I use BaGet as my local dev NuGet. I make changes and push a package to BaGet, then consume it. It gives me a possibility to test my work and make a merge request with a whole feature. This is fine. But also I need to have a possibility to have a local cache of our internal packages. Now I could not do it with my BaGet, cause it supports only open feed mirroring.

Describe the solution you'd like

• Supported authentication types should be investigated
• Additional parameters (like a credentials) should be supported by a configuration file (could be kinda polymorphic for different auth types)

Describe alternatives you've considered

In terms of BaGet I don't see any alternatives. I could use a local folder like a cache (but is needs some manual job) or another package manager

[mishamyte]

I would be happy to implement it after finishing #717