map: Add max_entries() for Map objects

[Jongy]

My use case is:

1. Read map's max_entries before loading, and resize max_entries according to some logic.
2. After map is loaded, access resized max_entries for logging etc.

For the sake of completeness I added max_entries on MapCore and implemented on MapHandle too, although I don't use this flow (I can add just on Map instead, or instead implement it with bpf_obj_get_info_by_fd to avoid increasing the struct's size)