Allow passing Authorization header with ENV variables to authenticate private modpacks

[dylanxyz]

Sorry if the title is not descriptive enough, english is not my main language.

Basically, packwiz-installer should be able to use an environment variable (or perhaps a command-line argument) to specify the Authorization header when making requests for modpacks hosted privately.

For example, in my use case, i'm hosting my packwiz modpack with Github on a private repo. This is because i only want my modpack to be accessible to a few people. Github allows me to generate an access token that gives read-only access to a specific repo, but the token needs to be sent in the Authorization header.

So the solution i'm thinking is for packwiz-installer to check if there is an environment variable, like PACKWIZ_HEADER_AUTH and add it when making request to download metadata files from the modpack.