Parallelize source hydrator manifest generation

[crenshaw-dev]

Summary

Today when the Source Hydrator generates manifests for a group of apps destined for the same target branch, it does so in series. We should do it in parallel.

Motivation

Hydrating in series is slow.

Proposal

Use an errgroup to run the hydrations in parallel. Use a context so that we fail fast if there's an error.

Eventually we should probably implement parallelism limits, jitter, etc. But for now just a simple errgroup will do.