feat(build): add off-by-default DPP_USE_PCH option in CMake

[Mishura4]

Adds an option to use private PCH when building DPP from source.
Mostly useful on MSVC, makes the build take 30 seconds instead of 5 minutes.
Useful for contributors mostly 👀 as this only affects DPP itself due to being private, and does not propagate to dependents.

This does not affect vcpkg.

• My pull request is made against the dev branch.
• I have ensured that the changed library can be built on your target system. I did not introduce any platform-specific code.
• I have ensured that all methods and functions are fully documented using doxygen style comments.
• I tested my commits, by adding a test case to the unit tests if needed
• I have ensured that I did not break any existing API calls.
• My code follows the coding style guide (if you are not sure, match the code style of existing files including indent style etc).
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight. Where I have generated this pull request using a tool, I have justified why this is needed.

[netlify]

✅ Deploy Preview for dpp-dev ready!

Name	Link
🔨 Latest commit	106f793
🔍 Latest deploy log	https://app.netlify.com/sites/dpp-dev/deploys/64e4bada12e4010008c505a9
😎 Deploy Preview	https://deploy-preview-793--dpp-dev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[Mishura4]

I could add it to the vcpkg CMake as well, I'm just not familiar with vcpkg so I'm not sure how I'd test that 🤔

[braindigitalis]

I could add it to the vcpkg CMake as well, I'm just not familiar with vcpkg so I'm not sure how I'd test that 🤔

yeah lets leave vcpkg alone. it works, and its fragile af. Lets not poke the hornets nest.