pkg/announce: make interface functions public and add tests

[embik]

What type of PR is this?

After refactoring pkg/announce in #3580, this is the first wave of tests to validate that the new packages work properly. I didn't want to have all tests in one PR (as that will result in a very large diff to review), and I was hoping to get some feedback on my approach before committing to work on additional tests.

In this PR:

• the functions on the implementation interface for k8s.io/release/pkg/announce.Announce are made public, because otherwise they can't be used for mock tests. I'm not sure why I chose them to be private in the first place.
• some light validation is added to the CreateForRelease method to ensure that empty data (empty changelog HTML or Go version) is caught.
• Reading the changelog file has been moved to the implementation interface, to eliminate the remaining direct filesystem call.
• Tests have been added that make sure creating an announcement fails for various failure scenarios.

Overall, CreateForBranch and CreateForRelease are very simple methods, but this adds test coverage nonetheless. If this is looking good, I will move on to the sub-packages in pkg/announce in future PRs.

/kind cleanup

What this PR does / why we need it:

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

[saschagrunert]

Awesome work, thank you!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: embik, saschagrunert

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