Rework CMake validation layer options and add flags for best practices and sync validation

[SaschaWillems]

Description

This PR adds two new CMake options related to validation layers:

• VKB_VALIDATION_LAYERS_BEST_PRACTICES
  Enables the best practices layer.
• VKB_VALIDATION_LAYERS_SYNCHRONIZATION
  Enables the synchronization validation layer.

These flags let developers easily toggle what part of the validation layers are active without having to e.g. use vkconfig from SDK.

Enabling one of these flags also implicitly enables validation layers. This was also retrofitted for the already existing VKB_VALIDATION_LAYERS_GPU_ASSISTED. So as soon as one of these flags is set, validation layers are enabled. This makes them less confusing to use.

General Checklist:

Please ensure the following points are checked:

• My code follows the coding style
• I have reviewed file licenses
• I have commented any added functions (in line with Doxygen)
• I have commented any code that could be hard to understand
• My changes do not add any new compiler warnings
• My changes do not add any new validation layer errors or warnings
• I have used existing framework/helper functions where possible
• My changes do not add any regressions
• I have tested every sample to ensure everything runs correctly
• This PR describes the scope and expected impact of the changes I am making

Note: The Samples CI runs a number of checks including:

• I have updated the header Copyright to reflect the current year (CI build will fail if Copyright is out of date)
• My changes build on Windows, Linux, macOS and Android. Otherwise I have documented any exceptions

[JoseEmilio-ARM]

LGTM, but curious, how is this necessary now, #245 suggested these messages should already have been visible? Oh, it looks like it required the user enabling some settings externally, whereas this PR enables them programmatically 👍

[gpx1000]

Small things to consider on this one. It's a good change and we need it.

[asuessenbach]

While introducing an option for the BEST_PRACTICES layer, we could also introduce an option for SYNCHRONIZATION_VALIDATION ?
Which would make implicit enabling of VALIDATION_LAYER even more important.

[SaschaWillems]

I deliberately didn't change how and when the general layer flag is enabled and just expanded the way that the GPU assisted layer works.

But sure, I could integrate all of your feedback and rescope the PR. Will take a look at that.