Include all submodules in the release notes.

[blakeli0]

We are bumping up minor version for every submodule in google-cloud-java on release, even if there is no code changes, it makes sense since we are at least updating the shared dependencies for every submodule. But what happens is that the release notes do not include every submodule, only the ones had fix/feat PRs, so customers don't know what actually changed if they only care about a certain module. e.g. java-iam-admin has no code changes in the last two months but we released three minor versions(3.1.0, 3.2.0, 3.3.0), and it is not mentioned in the aggregated release notes https://github.com/googleapis/google-cloud-java/releases.

Ideally, we should include all submodule in the release notes, and provide information like These modules only have dependencies update or similar ways to submodules only have dependencies update.

[suztomo]

@chingor13 Do you receive similar requests from other monorepos? If not, what do you think about pluggable release note function in Release Please? (I want that)