Need to allow customization of when and how auth tokens are fetched

Key Vault will use the `BearTokenCredentialPolicy` but needs to customize some behavior of it, like being able to empty the body of an unauthenticated request to avoid leaking secrets.

See <https://github.com/Azure/azure-sdk-for-rust/pull/1726#pullrequestreview-2243609123> for context.