Add samples demonstrating implementing TokenCredential directly with ADAL / MSAL

[schaabs]

There is a common scenario that Apps which already are using ADAL or MSAL to authenticate to services such as O365 also want to use the same instance of AuthenticationContext or PublicApplicationClient to authenticate to azure services. We should provide samples which demonstrate how to wrap an ADAL or MSAL client in a custom TokenCredential implementation to accomplish this.

I have a start to a such sample in response to the user issue #8621

[christothes]

https://github.com/Azure/azure-sdk-for-net/blob/f75a67b7ca8575dd1041b0bfdad6235f4b2c0413/sdk/identity/Azure.Identity/samples/DefiningCustomCredentialTypes.md#authenticating-with-msal-directly