Add UTF8 String support to Readable/WritableSequentialData implementations

### Problem

Currently PBJ uses a mixture of code to work on UTF8 Strings in binary data. None of them have direct access to stored data so work via read/writeByte methods and hence will be slow. See `com.hedera.pbj.runtime.Utf8Tools`.

### Solution

Add UTF8 read write methods to `ReadableSequentialData` and `WritableSequentialData` implementations. Replace Utf8Tools usage with new methods. Is there a faster method than `new String(bytes,StandardCharsets.UTF_8);` used in `com.hedera.pbj.runtime.ProtoParserTools#readString`.

See https://github.com/real-logic/agrona and https://github.com/OpenHFT/Chronicle-Bytes for ideas for fast tricks.

We need to stay very compatible to Google Protobuf interpretation of UTF8, so we are byte and byte compatible.