Include announcement posts as part of image publishing

[mthalman]

Consider including posting of announcements associated with the release as part of an image publishing pipeline run. Today posting of announcements is done manually. But with automation, the announcement will be posted as soon as images are available, freeing up an engineer from monitoring and responding.

One way of doing this is to prep a file containing the announcement content in a GitHub gist that can be referenced when running the pipeline. This mechanism already exists for the source build release pipeline.