b/316247020 Refactor class structure

[jpassing]

• Use a more consistent naming scheme
• Split notions of:
  • Entitlement catalog, which defines who has which entitlements
  • Entitlement activator, which provisions access
• Allow more than one implementation of each

This refactoring facilitates the following:

• Adding alternate catalogs
• More efficient activation of multiple entitlements in a single request