Introduce vsag::Engine to manage resource instead origin vsag::Factory

[LHT129]

Regarding the management of Allocator, we are currently quite confused, especially in various special scenarios, such as static global default allocator, multiple indexes sharing one allocator, etc.

Therefore, we plan to introduce vasg::Engine, a new data structure, to manage resources such as allocator, thread pools, etc. Index generation can only be completed by vasg::Engine, so that we can ensure that all resources are valid before the vsag::Engine shutdown, and the resources sharing problems can also be solved very well.

Of course, the resources required for vasg::Engine initialization can be imported from outside. The default resource implementation can be provided by vsag if the resource is not imported external.

The following PR will solve this problem:
#194
#209