cpp: CMake build

[ethanmusser]

The C++ build README.md calls out the third-party olympus-robotics/mcap_builder project for building with CMake.

mcap/cpp/README.md

Lines 95 to 97 in fcd77ea

	### CMake
	For using MCAP with CMake, the third-party [olympus-robotics/mcap_builder](https://github.com/olympus-robotics/mcap_builder) repository provides a helpful wrapper.

olympus-robotics/mcap_builder is quite a simple CMake wrapper that does the following:

1. clones this repository using FetchContent
2. optionally finds the lz4 and zstd depedndencies
3. creates a library target with a one-line source file that includes the <mcap/mcap.hpp> header
4. installs/exports that library target.

This is some fairly typical CMake boilerplate and is not particularly complicated. Why not support this as a first-class feature in the project itself? CMake is a very common build tool.

Instead of a wrapper library as in olympus-robotics/mcap_builder, the CMake build could simply find the lz4/zstd dependencies (optional), add an INTERFACE target, and then install/export that target.

Is there some reason that I am not considering for why a CMake build is not provided?

Thanks for reading!

[ethanmusser]

Note some relevant discussion from @james-rms in #1442 (comment).