[Identity] Enable @azure/identity to build for the browser

[daviwil]

This change makes some tweaks to @azure/identity to enable it to build for and be used successfully in the browser. This is the first step toward adding user authentication in the browser for Preview 2.

The largest part of the change is breaking out all of IdentityClient's authenticate* methods into the individual Credential classes that use them so that they can be conditionally included in the browser build. I'm also stubbing out ClientCertificateCredential, ManagedIdentityCredential, and EnvironmentCredential entirely because they rely on the Node.js environment and modules.

I've tested that this works successfully using an example project that pulls @azure/identity in to a Webpack build that runs in the browser. I'll send a separate PR for that example project once it's ready.

[daviwil]

OK, I think this is looking much better now. Question for @bterlson: should we be shimming Node-only classes to undefined like I'm doing here? I saw this being suggested in a storage PR and adopted the same strategy, but I'm not sure what the end result looks like to the user.

In the generated browser bundle I see the ClientCertificateCredential name being exported as undefined, so it seems like the user will be able to import it but will fail to be able to new it. Should there be a minimal shim that throws an exception with a helpful message?

[bterlson]

In the storage case it worked because we were shimming functionality that would disappear completely in the browser build. Since this is shimming exported surface area, it's probably a good idea to give good errors if possible!

[sadasant]

Looks good!! Can't wait to try it.

[daviwil]

OK, added class definitions for the non-supported credential types which throw error messages when browser use is attempted. Now I think this is ready for final review :)

[HarshaNalluru]

Is this change required for the other sdks?

[daviwil]

Probably so, I'll open a separate PR to make that change everywhere then we can decide whether to merge it.

I've addressed the comments in earlier changes

[daviwil]

Going to merge this now to unblock other work. If anyone has any further comments I'll be happy to open another PR to address them. Thanks all!