Refactor pkg/announce package to adhere to the interfaces pattern

[xmudrii]

We follow the pattern of using interfaces to provide an option to mock parts of the implementation, e.g. see https://github.com/kubernetes/release/blob/a00489762c923fe58d7e02ea67e06c54123594e6/pkg/release/archive.go

The pkg/announce package doesn't follow this pattern at the moment and it's hard to write tests for it. We would like to have that tests for this package to make it easier to accept changes such as #3559

To make writing tests possible, we should refactor the package to adhere to the "interfaces pattern" that we use in other packages. This issue assumes only refactoring the package, writing tests is the second step once the package is successfully refactored.

From the high level overview, we need the following:

• Announce struct that contains announceOptions and announceImpl
• announceOptions struct that contains options and parameters for relevant functions
• announceImpl interface that contains functions that should be mocked in testing (e.g. reading files...)
• functions should be refactored to be have Announce or announceImpl as a receiver (e.g. func (a Announce) ... or func (impl announceImpl) ...)

[embik]

I'd be interested in working on this.

[xmudrii]

Thank you! I'll assign the issue to you
/assign @embik