git-archive, RPC: add `Submitter` to available data
- [x] I have summed up the feature in concise words in the [Summary](#summary) section.
- [x] I have completely described the feature in the [Description](#description) section.
- [ ] I have completed the [Blockers](#blockers) section.
### Summary
Add a `Submitter` field to AUR metadata.
### Description
Having the `Submitter` available next to the `Maintainer` allows to do some useful analytics, e.g. [finding out all packages which are maintained but not submitted by a certain account](https://github.com/BioArchLinux/Packages/issues/79). Currently this can only be approximated by heuristics on `FirstSubmitted`.
This seems like an easy change for the git-archives; for the RPC this could be part of v6.
issue