feat(main): add forward SBPP_OnClientPreAdminCheck

Description

Add a forward SBPP_OnClientPreAdminCheck to control when OnClientPostAdminCheck will be called. We can then return a boolean to block the commands that will trigger OnClientPostAdminCheck:

RunAdminCacheChecks(i);
NotifyPostAdminCheck(i);

Motivation and Context

We are using https://github.com/R1KO/Vip-Core and we need to wait for both sourcebans-pp and vip-core to check if the player has a vip and/or an admin status so that we can trigger OnClientPostAdminCheck with the proper sourcemod admin flags

How Has This Been Tested?

In use on https://nide.gg for about two years implemented throught this module:

https://github.com/srcdslab/sm-plugin-VIP_SourcemodFlags/blob/master/addons/sourcemod/scripting/VIP_SourcemodFlags.sp#L140

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

[x] My code follows the code style of this project.
[ ] My change requires a change to the documentation.
[ ] I have updated the documentation accordingly.
[ ] I have read the CONTRIBUTING document.

sbpp / sourcebans-pp