ci: dispatch clas12-validation workflow runs

[c-dilks]

Adds a workflow to dispatch clas12-validation workflow runs.

Since this is a PR from a fork, the PAT is not accessible and the dispatch job will fail until this PR is merged.

To see an example working dispatch, see this equivalent PR against my fork, along with a successful workflow dispatch run; follow the "Workflow URL" link in the run's summary table to see the dispatched clas12-validation run.

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Unfortunately, a PR from a fork will never* have a successful validation dispatch, because of the inaccessible PAT; one possible workaround is having a contributor from a fork open a test PR on clas12-validation, manually changing the default coatjava fork and branch here, but that's not ideal.

*well.. unless we use the dangerous pull_request_target trigger, which might be okay as long as we require approvals for running workflows for PRs from forks. IIRC the only secret we have is the PAT for clas12-validation dispatches, but if we enable pull_request_target, we need to be very careful if we forget about this and add another secret, especially if such a secret grants write access somewhere.

[c-dilks]

Note: this uses my fork of trigger-workflow-and-wait so that we may use convictional/trigger-workflow-and-wait#90