Use designated initializers for ChainstateManager::Options and move into kernel:: namespace

Summary:

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.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15312