Overloads for flattened ByteString method args

[bdhess]

It would be nice if flattened RPC calls had overloads that accepted standard Java types (i.e. a Java byte[] and/or ByteBuffer) instead of always requiring a protobuf ByteString. The latter ends up leaking an implementation detail to callers.

As an example, in Cloud KMS we currently have:

String textRaw = "The quick brown fox jumps over the lazy dog!";
byte[] textBytes = textRaw.getBytes(StandardCharsets.UTF_8);
EncryptResponse encryptResp = client.encrypt(keyPath, ByteString.copyFrom(textBytes));

It would be nice if the final line could be

EncryptResponse encryptResp = client.encrypt(keyPath, textBytes);

or

EncryptResponse encryptResp = client.encrypt(keyPath, ByteBuffer.wrap(textBytes));

[bdhess]

Withdrawing this request, for the same rationale as discussed in the linked .NET issue.