feat: Adding PbjReader

New `PbjReader` class: varint/fixed-width/byte/boolean/string/bytes reads over a reusable ~16KB buffer, constructible from byte[]/ByteBuffer/Bytes/InputStream/ReadableSequentialData. Uses a sticky error code instead of throwing on every read.

**Benefit:** avoids per-call buffer allocation and checked-exception-based control flow on the hot read path.

PR: hashgraph/pbj#907

The issue describes a new PbjReader with reusable buffering, multiple input types, primitive reads, and sticky errors, but names no files or tests. Start by reviewing PR hashgraph/pbj#907 and the existing parser entry points. Done means the class supports each listed input and read type without per-call buffer allocation or repeated exceptions.

Written by the indexing model from the issue text.