`SendEachAsync` vs `SendAllAsync`
1. `SendEachAsync` sends one HTTP request to V1 Send endpoint for each
    message in the list.
   `SendAllAsync` sends only one HTTP request to V1 Batch Send endpoint
    to send all messages in the list.
2. `SendEachAsync` calls `Task.WhenAll` to wait for all
    `httpClient.SendAndDeserializeAsync` calls to complete and construct
    a `BatchResponse` with all `SendResponse`s.
    An `httpClient.SendAndDeserializeAsync` call to V1 Send endpoint
    either completes with a success or throws an exception. So if an
    exception is thrown out, the exception will be caught in `SendEachAsync`
    and turned into a `SendResponse` with an exception.
    Therefore, unlike `SendAllAsync`, `SendEachAsync` does not always throw
    an exception for a total failure. It can also return a `BatchResponse`
    with only exceptions in it.

`SendEachForMulticastAsync` calls `SendEachAsync` under the hood.

SendEach() and SendEachForMulticast()

…343)

* Implement `SendEachAsync` and `SendEachForMulticastAsync`

`SendEachAsync` vs `SendAllAsync`
1. `SendEachAsync` sends one HTTP request to V1 Send endpoint for each
    message in the list.
   `SendAllAsync` sends only one HTTP request to V1 Batch Send endpoint
    to send all messages in the list.
2. `SendEachAsync` calls `Task.WhenAll` to wait for all
    `httpClient.SendAndDeserializeAsync` calls to complete and construct
    a `BatchResponse` with all `SendResponse`s.
    An `httpClient.SendAndDeserializeAsync` call to V1 Send endpoint
    either completes with a success or throws an exception. So if an
    exception is thrown out, the exception will be caught in `SendEachAsync`
    and turned into a `SendResponse` with an exception.
    Therefore, unlike `SendAllAsync`, `SendEachAsync` does not always throw
    an exception for a total failure. It can also return a `BatchResponse`
    with only exceptions in it.

`SendEachForMulticastAsync` calls `SendEachAsync` under the hood.

* Add integration tests for the batch-send reimplementation:
SendEach() and SendEachForMulticast()