Extract Activitypub logic into separate library (#2288)

Extract Activitypub logic into separate library (#2288)

* Create example for apub lib

* some rewriting of apub lib

* Add LocalInstance struct for apub lib to avoid using Lemmy Settings

* Move ActorType trait to lemmy_apub, because its not needed in library

* Use reqwest_retry instead of custom impl, dont specify timeout on every send()

* Some improvements to example

* Moved inbox handling to library

* bug fixes

* Move context and serde helpers into library

* wip: example changes

* Add lemmy_utils feature to build only LemmyError

* Rename to activitypub_federation

* Remove lemmy_utils dep from activitypub_federation using generic error type

* Finish activitypub example

* Cleanup and fix tests

* Reorganize library files

* Remove ApubObject.to_tombstone()

* Extract activitypub library into separate git repository