Allow custom workflow snippets

[alvarosanchez]

There are some repositories, like micronaut-oracle-cloud, that for CI need some custom secrets to be passed as environment variables. Our current setup doesn't allow this, and they either take the common workflow file (which won't work) or opt-out for that file completely.

One solution could be to allow workflow snippets, so that the sync process would compose the final workflow file inserting those snippets at certain places.

The repositories interested in this would have workflow snippets .github/${workflow}-setup.yml and .github/${workflow}-cleanup.yml, eg:

• .github/gradle-setup.yml
• .github/release-cleanup.yml
• etc.

The sync script would inject setup snippets after checkout, and cleanup snippets at the end.

In the example of micronaut-oracle-cloud, the snippet would export the secrets as environment variables.