Turn the Federation class into an interface

     -  Removed `Context.getActorKey()` method.

        Use `Context.getActorKeyPairs()` method instead.

 -  The `Federation` is no more a class, but an interface, which has been

    planned since version 0.10.0.  [[#69]]

     -  `new Federation()` constructor is removed.  Use `createFederation()`

        function instead.

     -  Removed `Federation.sendActivity()` method.

        Use `Context.sendActivity()` method instead.

     -  Removed `Federation` class.

     -  Added `Federation` interface.

     -  Removed `FederationParameters` interface.

 -  Added more log messages using the [LogTape] library.  Currently the below

    logger categories are used:

     -  `["fedify", "webfinger", "server"]`

[#69]: https://github.com/dahlia/fedify/issues/69

Version 0.12.0

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