1. Deprecate sendAll and sendMulticast
2. Add dummy implementation for sendEach and sendEachForMulticast to avoid errors reported by api-extractor

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

`sendEachForMulticast` calls `sendEach` under the hood.