fix(c/driver/postgresql): encode Arrow JSON for JSONB COPY

[BOKJUNSOO]

Summary

PostgreSQL binary COPY represents a jsonb field as a version byte followed by
the JSON text. The PostgreSQL driver currently sends the physical string storage
of extension<arrow.json> unchanged, so PostgreSQL interprets the first JSON byte
as the version (for example, { becomes version 123) and rejects the row.

This adds a target-aware JSONB COPY field writer. When the Arrow source is
extension<arrow.json> and the existing PostgreSQL target column is jsonb, the
writer prefixes the JSON bytes with version 0x01 and includes that byte in the
field length. Other string inputs and PostgreSQL json targets continue to use
the existing writer.

The existing JSONB ingest regression test now verifies a successful round-trip,
including a SQL NULL value.

Testing

• pre-commit run --files c/driver/postgresql/copy/writer.h c/driver/postgresql/postgresql_test.cc (all applicable C/C++ checks passed)
• adbc-driver-postgresql-test (242 passed, 6 skipped)
• adbc-driver-postgresql-copy-test (73 passed)
• Loaded 100 extension<arrow.json> values from the reported sample Parquet file through the locally built driver into a PostgreSQL jsonb column without a compatibility shim (100 inserted and validated)

Depends on #4499 for resolving existing PostgreSQL target column types during
COPY writer selection.

Closes #3293

[lidavidm]

Thanks, looks pretty straightforward.

[lidavidm]

CC @Mandukhai-Alimaa

[Mandukhai-Alimaa]

LGTM.