Add release CI

[yokawasa]

Description

Add release CI where the following steps are executed after test CI (merged in PR https://github.com/st-tech/gatling-operator/pull/19):

• CI trigger: tagging (semantic ver like v0.0.1)
• Create release YAML (all-in-one manifest for Gatling Operator)
• build and push the image to GHCR
• create release and upload the release YAML

the way we trigger the release CI is simply to tag the release like this:

git tag -a v0.1.0 -m "v0.1.0"
git push origin v0.1.0

Relevant issue #3

Test

CI has been run with dummy trigger and dummy tag v0.0.1-alpha.2 successfully. See this below

1. test GitHub Actions run: https://github.com/st-tech/gatling-operator/runs/4624459901?check_suite_focus=true

2. created release: https://github.com/st-tech/gatling-operator/releases/tag/refs%2Fpull%2F23%2Fmerge

3. pushed images (ghcr.io): https://github.com/st-tech/fluent-pvc-operator/pkgs/container/gatling-operator

[yokawasa]

@niqniqniqq thanks for the review! Let me merge this and create a 1st gatling-operator release