Adding metadata to application CRUD group

[svix-gabriel]

This adds the metadata field to the application CRUD group.

Motivation

The metadata field is just a JSON dictionary with string only values. This field was added to our public API, but never to our open source solution. This PR corrects that.

Solution

We add the field to our /apps CRUD group. Metadata itself is a separate joined table, instead of a column on the apps field, because metadata can be quite large (up to 4096 bytes).

Because it's a separate model, the sea_orm queries end up being pretty tedious. I tried to keep things efficient and query for both records in a single round trip where possible, but in some places we needed two roundtrips.

A side effect is that the endpoints were starting to get fairly complicated, so I tried to offload a lot of the logic to the relevant db::models where appropriate to offset this.

Finally, the metadata field is entirely optional, but we default to rendering {} if it's unspecified. Additionally, we don't create empty metadata records. It ended up being much simpler to just pass around an applicationmetadata::[Model|ActiveModel] around, and intelligently upsert only IFF the underlying data wasn't empty.