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.

No implementation files or tests are named. Start by tracing the PATCH endpoint and current media-type handling, then review the linked JSON Patch and Cosmos DB references. Done means the vendored media type accepts the shown incr operation for a column path and produces the described partial SQL update.