[FEATURE] Added GetServers() to Inventory

Added new public member, GetServers(), to the Inventory class. Calling this method will return a JSON formatted string of all the servers in the database.
[BUGFIX] Fixed GetByType SQL injection

Wrapped $procedure's value with a real_escape_string call (MySQLi) to prevent any SQL injections.
[!!!][API] Replaced GetStoredProcedureName()

GetStoredProcedureName has been replaced with GetQuery() and now requires the full query to be returned instead of just the stored procedure name. This is a breaking change and all child classes must implement the new GetQuery() method. This modification has removed the reliance on stored procedures for child classes.