Authenticate via Visual Studio Code

[chlowell]

Add a credential to authenticate with the identity signed in to Visual Studio Code's Azure extension. This credential should be internal because it may be exposed to users only as part of an aggregate credential. The Azure extension uses keytar to store a refresh token, so the credential will need to make native calls to get that token. I expect msal-extensions has an interface for some of these calls (e.g. for macOS Keychain). Sharing authentication with the Azure extension means redeeming the refresh token it stored; AadClient has code for that.

Azure/azure-sdk-for-net#10979 has a complete cross-platform implementation in C#.

[xiangyan99]

See in #10840