Create an infrastructure for rego policies

[nadavgross]

In this issue we would want a "main" rego file that calls different policies from different files.

• create a rego file called main_policy.rego in the opa/policies path.
• its package should be called policies.
• this file should contain the allow and batch rules for each "set" or rules
• this file should import rules from other policies files that each implement a specific use-case for authorization.