Using a shared_ptr removes the need for manually calling
`libcamera_manager_release()` to drop the reference as it is done
automatically whenever the shared_ptr is destroyed or reset.
Previously, the "impl" object was never properly constructed or
destructed, but it more or less worked out since the memory was initialized
to zero bytes and each member had trivial constructors. Except std::shared_ptr,
but an all zero storage happened to be equivalent to a default constructed
shared_ptr.
However, there was the still the problem that the shared_ptr was never
destructed, so it kept the referenced `Camera` object alive, which lead
to memory leaks.
An additional, somewhat unrelated change is that the "props" struct
is removed, and the device identifier is now stored in an `std::string`.
The reason is that `CameraManager::get()` already takes a const std::string reference,
so an std::string must be constructed in any case, so we might as well
take advantage of that and use `std::string` in the "impl" object as well.
Furthermore, wrap the `impl` struct in an anonymous namespace
to avoid name resolution problems.
libcamera commit 1c4d4801850559d6f919eef5c2ffbaf7675dbc46
changed the return type of libcamera::ControlList::get()
to be std::optional<T>. Adapt the code to this change.
Fixes#2575
