Add cancelling the pipelinerun feature

[khrm]

Add cancelling the pipelinerun feature

Fix #12

Changes

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• Includes tests (if functionality changed/added)
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Release Notes

Now a pipelinerun can be cancelled.

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-cli-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/cmd/pipelinerun/cancel.go	Do not exist	94.4%

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-cli-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/cmd/pipelinerun/cancel.go	Do not exist	94.4%

[hrishin]

/ok-to-test

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-cli-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/cmd/pipelinerun/cancel.go	Do not exist	94.4%

[vdemeester]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: khrm, vdemeester

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[vdemeester]

/test pull-tekton-cli-unit-tests