PMM-11423: support for updating a service (#1590)

* PMM-11423: support for updating a service

* Refactor update of labels to a model

* Rename UpdateService to ChangeService

* Change API comments

* Split business and model logic

* Remove unused code

* Add tests for changing standard labels

* PMM-11546 remove scheduled backups when cluster changed (#1662)

Remove scheduled tasks on cluster changing.

* PMM-11423 fix conflicts

* PMM-11423 fix linter errors

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Co-authored-by: Denys Kondratenko <[email protected]>
Co-authored-by: Pavel Khripkov <[email protected]>
Co-authored-by: Alex Tymchuk <[email protected]>
Co-authored-by: Matej Kubinec <[email protected]>