Idea: provide update metadata for agent usage

When performing updates, [`dogswatch`](./184) is able to observe the cluster from the context of the orchestrated environment and on the individual hosts by way of the communicated annotations and Thar metadata. These details can be extended as needed to empower rich policy controls or to improve the stability of Thar hosts in any given cluster.

This is a list of ideas and interesting metadata that *can be* put to use immediately for a given update offered to/by the node (N.B. this list is not exhaustive, not prioritized, and not necessary today):

- version
- variant
- security brief
- impacted keys
- impacted versions
- build brief
- source
- timestamp
- migration brief
- impacted keys
- update phase group identifier

Aspirational data & niceties for feedback to improve stability/visibility:

- update reporting URL

Reporting for update decision making:

- success
- failure
- withheld (for update policy)
- withheld (for workload policy)

- self reference URL (remote copy) to update metadata

Helping to avoid overloading annotations with extensive data.