Turn the `Federation` class into an interface

[dahlia]

For now, it is difficult to hide the details of its implementation, as the Federation is a class. Like the relationship between the Context interface and the ContextImpl class, we should extract out the FederationImpl class from the Federation interface (which is currently a class).

• [x] Deprecate new Federation() constructor: https://github.com/dahlia/fedify/commit/22cef13ae2d2d00153898a368c941f9ae69ee7e7
• [x] Introduce createFederation() function: https://github.com/dahlia/fedify/commit/22cef13ae2d2d00153898a368c941f9ae69ee7e7
• [ ] Replace new Federation() constructor with createFederation() function
• [ ] Remove FederationParameters interface
• [ ] Rename Federation class into FederationImpl
• [ ] Extract out Federation interface from FederationImpl class
• [ ] Let ContextImpl and RequestContextImpl directly access to a FederationImpl instance