ci: .NET workflow tweaks

[tippmar-nr]

• Integrates with the test.yml reusable workflow, had to make a couple minor changes.
• Modified file header to use an SPDX-License-Identifier tag as suggested in a thread in #agent-dev, makes things much more compact
• Adds inputs for workflow_dispatch to optionally allow publishing an arbitrary version of the agent without requiring a release tag
• Only sets the latest tag when publishing from a release trigger or when request during manual run
• Updates workflow triggers to match other agent workflows, specifying paths to consider before triggering. This might make the check-modified-files workflow unnecessary.

[mrickard]

LGTM! Thank you @tippmar-nr !