Add composability/modularization to GH action templates

[vdice]

Currently, each supported language has their own set of GH actions. There is a decent amount of shared/duplicated yaml between these.

The goal of this issue is to consolidate common GH action yaml. A couple things to look into:

1. Consolidating common yaml, perhaps via common templates that all languages can build from
2. Are there any candidates for creating public GH actions? If so, these can be updated to consume them once available.

[itowlson]

Templates or blocks might be a good initial strategy. Our actions are very much in the pattern of "language determines build, registry determines publish" with the main touchpoint between the two being where to find the assets. So if we could create per-language and per-registry YAML blocks that we could just drop in at substitution time...?

Referencing or creating encapsulated GH actions would also be great - this could help developers putting together workflows by hand, or simplify understanding for devs editing the scaffolded workflows.