Make building examples optional

[jeongseok-meta]

This PR proposes an opt-out approach for building examples, allowing the package to exclude vsgXchange as a required dependency when examples are not necessary (e.g., during packaging by package managers).

[robertosfield]

This will be a theme we'll see with other VSG add-on libraries. Perhaps it may be worth looking at standardizing the convention we use for building examples.

Is there any convention used in other projects that are worth looking at as reference?

It might be that we can make examples default to off. Or have a general env var/CMAKE var that we can use when building the various libraries and pure libraries.

[jeongseok-meta]

I agree that standardizing the convention for building examples across VSG add-on libraries is a good idea.

I've researched existing conventions and found that using environment variables or CMake variables, such as BUILD_EXAMPLES, is common practice. However, it's worth noting that there isn't a specific standard from CMake for building examples. Many projects use the *_BUILD_EXAMPLES variable, but the default value is typically up to the author. We could use VSG_BUILD_EXAMPLES or VSGQT_BUILD_EXAMPLES, where the latter seems the current convention of the vsg add-on libraries for CMake options.

In this case, I'm considering making examples opt-out by default, allowing novice users to easily try out the examples without changing any options. This approach seems user-friendly and convenient.

Ultimately, the decision on how to handle building examples is up to you, and I'll await your feedback before proceeding with any changes!

[jeongseok-meta]

In any case, please feel free to directly modify this PR as needed :)