Add lib-gpgme to the supported system libraries

[vencakrecl]

Following the guidance in docs/extension-maintainers.md to open a discussion before adding a system library.

I'd like to propose adding GpgME (lib-gpgme) to PIE's supported system libraries.

Context: I'm adding PIE support to php-gnupg/gnupg, which is a wrapper around the GpgME library. The extension links gpgme dynamically, so being able to declare "require": { "lib-gpgme": "*" } and have PIE check (and optionally auto-install) it would improve the install experience.

Why it fits PIE's existing mechanism: GpgME exposes a standard gpgme pkg-config module, so it can be detected the same way as the other supported libraries:

$ pkg-config --print-provides gpgme
gpgme = 2.1.2

Package names per distro:

• apt (Debian/Ubuntu): libgpgme-dev
• apk (Alpine): gpgme-dev
• dnf/yum (Fedora/RHEL): pkgconfig(gpgme) (provided by gpgme-devel)

Ready branch: I have a branch prepared with the pkg-config detection entry, the auto-install definitions, a passing data-provider test case, and the docs table update, and I'm happy to open a PR if this is welcome:
https://github.com/vencakrecl/pie/tree/add-gpgme-library

Would this addition be welcome?