chore: use real structs in hoghook payloads

[bretthoerner]

Problem

The manual JSON destructuring was ugly, and @xvello told me about serde(flatten).

Changes

Use flatten to store extra fields, then reconstruct the JSON payload before inserting the job.

The downside of serde(flatten) is we were mutating the incoming payload, risking an unexpected change to the passthrough data. Instead, I've adjusted it to the best of the both worlds: we use structs to deserialize, but we do it only on a copy of the subset of data we care about -- that way the original incoming payload is totally unmodified.

Does this work well for both Cloud and self-hosted?

Yes

How did you test this code?

Existing tests covered all the edge cases.