[Identity] Reduce scope of native dependency "keytar"

[mikeharder]

The "Rush" build orchestrator recently added a dependency on @azure/identity, but they would like to avoid taking a transitive dependency on keytar which is a native module. (microsoft/rushstack#2492)

Other users have reported issues caused by the keytar dependency (example #9288). As a result of this issue, keytar was changed from a required to optional dependency (#10142).

This does make it possible to use @azure/identity without installing keytar, but the dependency is still problematic. NPM tries to install optional dependencies by default. They can be skipped by specifying --no-optional, but this isn't very useful in practice since every user installing the package (or anything that depends on it) would need to set this.

If the keytar dependency cannot be removed completely, perhaps @azure/identity could be split into multiple packages, so only the package(s) containing APIs requiring keytar would have the dependency. @azure/identity itself would become a meta-package to avoid a breaking change.

[octogonz]

We would also be fine with a design where the keytar object is passed to the APIs that need to use it. Something like:

const keytar = require('keytar');
const { DefaultAzureCredential } = require("@azure/identity");

const credential = new DefaultAzureCredential({
  keytarLibrary: keytar
});

This way the consumer of @azure/identity could decide whether they want the keytar dependency or not (or under what conditions they want to install it). In Rush's case I don't believe that we access any credentials from the OS key vault, so we don't need keytar at all.

CC @iclanton

[octogonz]

volta-cli/volta#906 (comment) is saying that Volta cannot be used with Rush unless we can eliminate the native dependencies.

[witemple-msft]

Closing in favor of tracking #14346

We'll handle this in the same way as we handle msal-node-extensions, since it's shaping up to be that we'll have a separate package that allows us to add these native dependency modules dynamically.