Extensibility of `PlatformProxyAdmin`

[georgercarder]

Currently, the PlatformProxyAdmin provide premium service to the controller, and factory but does not consider future modules added to the v1 platform may need to be managed by the PlatformProxyAdmin.

motivation: The new StrategyControllerLens is proxied by a TransparentUpgradeableProxy which will have the PlatformProxyAdmin set as it's admin. But currently the deployment and initialization of that proxy must be done separately from the PlatformProxyAdmin. It would make sense that new platform modules are handled uniformly by the PlatformProxyAdmin hub.

For extensibility, the new version of the PlatformProxyAdmin will have

• a mapping to the contracts it services
• a "spawn" function populating the mapping and creating new proxies
• an external calculateAddress
• a sensible "getter" for the serviced proxies.
• backwards compatibility with the prior PlatformProxyAdmin's controller and factory