Add pkgconfig generation to cmake

[wreedb]

Summary

This PR provides two pkg-config file templates, for openzl and openzl_cpp,
along with configure_file calls for them in the corresponding section of the cmake build.

Type of Change

Build/CI changes

Test Plan

I built the project using a few different CMAKE_INSTALL_PREFIX's to ensure the installation path was being handled correctly, as well as using pccritic from pkgconf to validate the installed .pc files, and ensure they weren't missing any major fields.

Test Configuration

• Compiler: GCC 16.1.0
• Build type: CMake v4.2.1
• Platform(s): amd64 Linux

I noticed there was a comment in the CMakeLists.txt noting intent to add support for pkgconfig files in the installation, and I happen to be very familiar with the format, so I decided I'd lend a hand.

The .pc.in files in the commit reside in scripts/, though this may not be the correct place for them. I initially tried to put them in build-scripts/cmake/ which felt like a much more sensible location, however that led me to notice this line in the .gitignore:

build-*/

Which... I imagine must be a mistake, as this matches the build-scripts/ directory, which prevented me from adding them to that location.

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