[Identity] Azure SDK credential built on msal-node

[sophiajt]

Current Status (2020/09/10)

We are now moved onto msal-node for the InteractiveBrowserCredential for Node and for DeviceCodeCredential.

Next steps:

• add support for the cache and silent flow (blockers at [Identity] Blockers for msal-node + cache adoption #10804)
• support configurable http pipeline through MSAL

Is your feature request related to a problem? Please describe.
When msal-node becomes available, we can leverage their new capabilities by moving Identity to use msal-node directly rather than using the hand-made credentials of the same type.

Describe the solution you'd like
The first credential we could use, once available, is the auth-code credential. Following the design of the Python Identity library, we could call into the user's browser, allowing them to login and request an auth code. Once the auth code is made available, it can be passed to msal-node for processing and caching.

As this matures, we can move to other capabilities, like durable caches, device-code, and more.

Note: we'll need to coordinate with MSAL on ensuring that the HTTP transport layer is pluggable, so that we can use the standard Azure SDK layer, which will allow uniformly-configured proxy, retry, logging, distributed tracing, and possibly more in the future.

Merging with a duplicate issue:

Currently Azure.Identity for JS does not have feature parity with Azure.Identity for .NET/Java/Python due to the unavailability of a supported version of MSAL for NodeJS.

This work item tracks catching JS up to feature parity using the beta MSAL for NodeJS

shipping a public preview of Azure.Identity for JS using the first public beta MSAL for NodeJS
Reviewing and unifying the usage of MSAL / MSAL Cache, per the work done last semester in the other 3 primary languages
Adding Azure Identity Support for MSAL Configurable Token Refresh
Adding UserAuthentication API
This work will be kept in a separate feature branch until MSAL for NodeJS reaches a supportable / go-live level of quality, at which point this work can merge back to master and ship GA.