Allow defining required privileges to bypass protection checks

[S-S-X]

Allow defining required privileges to bypass protection checks for some nodes, this would allow defining administrative functions for registered tools.

Possible functionality:

• Option to require verification each time protection bypass is used
• Separate protection bypass settings for reading and writing
• Allow overriding default protection checks with before_read / before_write callbacks

[S-S-X]

Currently metatool:get_node does protection checks here https://github.com/S-S-X/metatool/blob/master/metatool/api.lua#L226-L230
Problem is that it does not know anything about operation and cannot therefore do selective allow or deny.

Currently flow goes like this:
item used -> minetest default on_use callback
-> metatool:on_use(itemname_clean, unpack({...})) api.lua#L46
-> metatool:get_node(tooldef, player, pointed_thing) api.lua#L91

Adding custom tool.is_protected(pos, player, node) callback for tool would be easy because all information for that is already available, that however wont make difference between read and write operations.

Better approach could be to provide node.can_read(pos, player, node) or tool.can_read(pos, player, node) and similar can_write methods.
This requires moving player:get_player_control() handling to be done before calling metatool:get_node method and metatool:get_node could then get information about operation that would be applied if node and position is returned.