Consistent CMake coding standards across projects

[feltech]

Ensure CMake style is consistent across projects. Note that linting is handled in a separate issue: #1132

This may involve building out a common set of CMake utilities.

But more likely just means that similar CMake tasks are accomplished in a similar way across projects, so that it's easier to grok one project given familiarity with another, and easier to copy-paste and customise snippets from one project to another.

[elliotcmorris]

Our cmake is a deliverable because people need to build elements of the ecosystem, often together, to suit whatever workflow they're going for.

[feltech]

Inconsistent styling does come up in reviews too e.g. https://github.com/OpenAssetIO/OpenAssetIO/pull/1348#discussion_r1678140343

Perhaps cmake-lint can enforce more than we've currently got it configured for?

Otherwise we can at least document as part of our style guide, so we can point to it when it comes up in a review.