authprovider

This package is mostly copy pasted from buildkit main branch https://github.com/moby/buildkit/tree/master/session/auth/authprovider We need it for 2 reasons:

• Pass a custom config (instead of loading the default one)
• AuthProvider takes a struct instead of an interface which makes it challenging to customize even if we pulled the main branch as a dependency.

Changes:

• NewDockerAuthProvider takes interface instead of struct
• Seed is hard coded to temp directory instead of config file path
• Everything in config.go is our code (not from docker)
• Added some error handling in tokenseed to satisfy GolangCI linter

From Docker:

• All logic in tokenseed
• Everything in authprovider.go except the Config interface and NewDockerAuthProvider changes mentioned above