feat: Adding PbjWriter

Write-side counterpart to PbjReader: fixed-width/varint(+zigzag) writes, plus a placeholder/patch mechanism for filling in length prefixes after writing content, avoiding a two-pass encode for short strings.

**Benefit:** same buffer-reuse win as PbjReader, applied to writes.

PR: hashgraph/pbj#909

Start by reading the existing PbjReader and the PR referenced in the issue to understand the intended writer structure. The work is done when PbjWriter supports fixed-width, varint and zigzag writes, plus placeholder patching for length prefixes without a two-pass encode.

Written by the indexing model from the issue text.