generate release notes for delta since latest release of each chart

[qrkourier]

Change the way --generate-release-notes works.

If --release-notes-file exists, then --generate-release-notes is ignored. It was previously appended after the release notes file's contents in the release description.

Else if --generate-release-notes, then use the GitHub API to generate notes for the delta between the latest release of the chart and the current commit which will be the next release.

Else, as before, use the chart metadata description as the release description.

resolves helm/chart-releaser-action#44

---

The proposed change allows cr to focus release notes on the relevant pull requests since the latest release of the same chart. Combined with a .github/release.yml, the generated notes can be further narrowed to ensure they only include pull requests for the specific chart. This requires labeling PRs which can be done automatically based on changed files in a pull request or manually.

I'll provide an functioning example using these elements before marking this PR as "ready for review."

[qrkourier]

This Chart Releaser change creates release notes like these with GitHub's built-in notes generator: https://github.com/openziti/helm-charts/releases/tag/ziti-controller-1.0.13

As before this change, the --release-notes-file is used if it exists instead of GitHub's notes generator.

GitHub's notes generator is configured by .github/release.yml like this: https://github.com/openziti/helm-charts/blob/ziti-controller-1.0.13/.github/release.yml

GitHub's notes generator keys on PR labels like notes:ziti-router that are automatically added based on changed files by this labeler action: https://github.com/openziti/helm-charts/blob/ziti-controller-1.0.13/.github/workflows/release-notes.yml#L19

Chart Releaser is executed by chart-releaser-action, a shell action I modified to accept the existing cr --generate-release-notes: https://github.com/openziti/helm-charts/blob/ziti-controller-1.0.13/.github/workflows/release-charts.yml#L44 uses https://github.com/netfoundry/chart-releaser-action/blob/v0.0.3/action.yml#L59