Add support for PostgreSQL 16, 17, and 18

[odie5533]

• Updated pgvector to v0.8.1
• Implement separate binary packages for PostgreSQL versions 16, 17, and 18
• Each package has its own pyproject.toml, Makefile, and README
• Packages are installable independently but designed to work with main package
• Modified _commands.py to search for binaries in pgserver_binaries/ namespace
• Python code auto-detects which binary package is installed

This provides pre-built wheels for all supported PostgreSQL versions while keeping download size reasonable (users only get the version they need).

# PostgreSQL 18 (latest, default)
pip install pgserver

# PostgreSQL 16
pip install "pgserver[pg16]"

# PostgreSQL 17
pip install "pgserver[pg17]"

Check which version is installed:

import pgserver
print(f"PostgreSQL version: {pgserver.INSTALLED_POSTGRES_VERSION}")

Wheels available at https://github.com/odie5533/pgserver/actions/runs/19176832696

[odie5533]

@orm011 Please take a look when you can!