Add API to specify which applications can share credentials

[craxal]

Summary

I don't think this is a problem for Windows or Linux, but Keychain in macOS only allows applications to access credentials they set themselves by default. The OS does have an API that allows you to grant permission to other applications access to their credentials, but keytar has no way of doing that.

Motivation

It's very common for applications to communicate with other applications to accomplish work. Sometime applications need to share sensitive information, such as credentials. For example, a cloud storage management application may want to use a program that specializes in transferring data. The management and transfer programs will need to pass credentials to each other.

Describe alternatives you've considered

You can write your own native node module (bleh!). So why not just make this a part of keytar? I'm willing to share what I have.