Add a composer.json with the `php-ext` type and metadata required by PIE
(https://github.com/php/pie), the modern replacement for PECL, so the
extension can be installed via `pie install php-gnupg/gnupg`.
The `configure-options` mirror the flags exposed by config.m4
(`--with-gnupg`, `--with-gpg`). Windows is excluded via
`os-families-exclude` since GpgME builds supported by PHP are unavailable
there. Document the PIE install method in the README.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
