* Add support for link type between issues

Fixes #847

* Create AbstractIssue

Fixes #883

Fixes #887

Fixes #936

Co-authored-by: Ипполитов Вадим Дмитриевич <vippolitov@poidem.ru>

Co-authored-by: Jérémie Bresson <dev@jmini.fr>

Co-authored-by: Michael Fitoussi <16884890+mifitous@users.noreply.github.com>

* Added ReleaseLinks API (#639)

* Deprecate the "external" attribute in Link

* Add tests

* Rename to ReleaseLinksApi and add the getter in GitLabApi

---------

Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

* Added hook and fetch capabilities

* Fix tests

* Remove superfluous formatting

Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

Fixes #889

* Add ExternalStatusCheckApi

Fixes #919

* Add ExternalStatusCheckEvent

* Fix javadoc

Co-authored-by: Jérémie Bresson <dev@jmini.fr>

---------

Co-authored-by: jason <jason.irwin@idoxgroup.com>

Co-authored-by: Vladimir Krumov <vladimir.krumov@paysafe.com>

Adding branches_to_be_notified to SlackService and deprecating notify_only_default_branch as per
https://docs.gitlab.com/ee/api/services.html#createedit-slack-service

- readme_url
- can_create_merge_request_in
- import_status
- ci_default_git_depth
- remove_source_branch_after_merge
- auto_devops_enabled
- auto_devops_deploy_strategy
- auto_devops_enabled
- statistics.packages_size
- shared_with_groups.group_full_path

fix #613 #632 #667

Added option to preserve underscores in the JacksonJsonEnumHelper to fix TargetType.MERGE_REQUEST value. (#615)