Dispatch release workflow from CI workflow

[lokst]

This updates our workflows to dispatch Release from the CI workflow.

A similar change was done for the dnsimple-java repository: dnsimple/dnsimple-java#218

Currently the Release workflow is directly triggered upon a tag push but as discussed here, it relies on a third-party action to validate that the CI workflow ran successfully for the tagged commit.

With the changes in this PR, we will use workflow_run to make the Release workflow run automatically when the CI workflow completes, with the following necessary accompanying changes:

• Trigger the CI workflow upon a tag push that matches a valid pattern
• We introduce a validate-tag job to the Release workflow, that validates that the CI workflow was triggered from a push of a valid tag, and only allows the publish job to run if the validation succeeds. This is necessary because the CI workflow can be triggered by other conditions including a push to main branch.

Belongs to https://github.com/dnsimple/dnsimple-engineering/issues/361