Partial document update based on JSON Patch

Problem

Currently there's no way to do a partial document update.

• https://github.com/PostgREST/postgrest/issues/465#issuecomment-1198884375 discusses json patch but there are certain operations (like test) which don't make much sense given our horizontal filtering.
• An alternative is proposed on https://github.com/PostgREST/postgrest/issues/465#issuecomment-1199858002, but it's flexibility makes it harder to implement. Additionally it's not isolated to a single content type.

Solution

Use a vendored media type based on json patch where we support the operations we lack. As prior art, azure cosmosdb json patch (also see here) does a similar thing and adds support for increment and set, which are not defined in the JSON Patch RFC.

For starters, we can add support for increment. Like so:

PATCH /items?id=eq.1
Content-Type: application/vnd.pgrst.json-patch

[
  { "op": "incr", "path": "/inventory", "value": 10 },
]

Path would refer to any column and the generated SQL would be like UPDATE items SET inventory = inventory + 10.

Later on we can extend this to add other operations, like mult, div, concat, etc.