Proposal add PerformService privilege

[Joseph Reynolds]

This is a proposal to add an OemOpenBMCPerformService privilege to BMCWeb.

See https://redfishforum.com/thread/397/redfish-direction-update-eeproms
As mentioned in the Redfish forum thread, the use case is that some 
OpenBMC use cases require isolating manufacturing and service functions 
away from the customer/admin (including updating FRU serial numbers, and 
updating a permanent MAC address), and this is a Redfish compatible way 
to do it.

The work items would be like:
- Add this OEM privilege to the base BMCWeb implementation.
- Identify URIs that we need to be able to isolate away from 
customer/admins.  Then modify the privilege mapping to require this 
privilege to PUT to those URIs.
- Add this privilege to the Administrator role (but not Operator or 
ReadOnly).
- Document how to isolate these operations.  Specifically, remove this 
privilege from Administrator, and create a custom OEM role that has this 
privilege

What do you think?
- Joseph