add azure_identity::new_credential(), SpecificAzureCredential, AppServiceManagedIdentityCredential, VirtualMachineManagedIdentityCredential

[cataggar]

This adds azure_identity::new_credential(), SpecificAzureCredential, AppServiceManagedIdentityCredential, and VirtualMachineManagedIdentityCredential.

• Similar to azure_core::new_http_client(), azure_identity::new_credential() is a function that simplifies the most common use cases.
• SpecificAzureCredential fixes add SpecificAzureCredential Azure/azure-sdk-for-rust#1503. It provides an easy and fast way to specify a specific credential type at runtime using an AZURE_CREDENTIAL_TYPE environment variable.
• AppServiceManagedIdentityCredential and VirtualMachineManagedIdentityCredential are split off of and make use of ImdsManagedIdentityCredential. This fixes split up ImdsManagedIdentityCredential Azure/azure-sdk-for-rust#1495.

Most all of the credentials have a create(options: impl into<TokenCredentialOptions>) -> Result<Self> now. This is similar to how C# has TryCreate shown in Azure#1495. They are able to check environment variables or files to see if they can be created, something milliseconds fast. The main exception is VirtualMachineManagedIdentityCredential, which does not have anything quick to check, so it is disabled by default. This is a breaking behavioral change that allows the default credentials to be faster by default. You can still opt in to using them by setting AZURE_CREDENTIAL_TYPE to virtualmachine. You can also include them using the DefaultAzureCredentialBuilder.

There are a few changes to make unit testing possible and easier. The azure_core::NoopClient is exposed behind a test feature. I create azure_identity::env::Env for access to environment variables.

[cataggar]

Published to Azure#1532.