Add job to run the release and sign the release with cosign

[cpanato]

• add a job to cut a release on-demand using workflow_dispatch
• sign the artifacts with cosign

Fixes: #403

A rehearsal of the release running with GitHub actions and the signing part

• https://github.com/cpanato/mage/releases/tag/v1.99.99
• https://github.com/cpanato/mage/runs/5662096376?check_suite_focus=true