Move ChainstateManagerOpts into kernel:: namespace
It should have been there in the first place.
Use designated initializers for ChainstateManager::Options
This wasn't available at the time when ChainstateManager::Options was
introduced but is helpful to be explicit and ensure correctness.
This is a backport of core#25607
Note that designated initializers are a c++20 feature, but are supported out of the box with C++17 by all supported gcc and clang versions.
For Visual Studio 2022 it is supported with /std:c++20 language mode.