Co-authored-by: Clayton Walker <cwalker@sofi.org>

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Co-authored-by: Salamon <aleksey.a.kryuchkov@raiffeisen.ru>

* feat(scheduled-pipeline): makes scheduled pipelines runnable

* Update src/main/java/org/gitlab4j/api/PipelineApi.java

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

* fix(scheduled-pipeline): change post()-call

* Update src/main/java/org/gitlab4j/api/PipelineApi.java

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

* Add missing import

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Co-authored-by: Jérémie Bresson <dev@jmini.fr>
Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>
Co-authored-by: Ruben Vitt <dev@rubeen.one>

Renamed modifyPipelineSchedule() to updatePipelineSchedule() to follow library naming conventions (#318).

* pipeline schedule support
* add test for json structure of pipeline-schedule