ci: add workflow disptach to test cases

Currenlty the workflows tests-cluster-chainsaw.yaml, tests-operator.yml and tests-plugin-barman-cloud.yaml Only runs on a pull request.
There shall be a way to use it on demand where we can run while adding editing test cases on CI.

Proposal: add workflow dispatch on each actions mentioned above.

Start by reading the trigger sections in tests-cluster-chainsaw.yaml, tests-operator.yml, and tests-plugin-barman-cloud.yaml. Done means each workflow can be started on demand as well as on a pull request; verify the manual dispatch option is available for all three.

Written by the indexing model from the issue text.