Add context menu option to set cell to default value

[seancolsen]

Current behavior

• Some columns may have dynamic default values defined in Postgres, for example to set a timestamp to the current time, or to set a number to a random value.

• The user is able to set a cell to the default value when adding a new record, but not after the record exists.

Desired behavior

• The cell context menu (which already contains an option to set the cell to NULL) contains an option to set the cell to DEFAULT. The menu entry also displays the Postgres expression to which the default value is defined.

Implementation

• If we want this feature, we need to decide how to convey DEFAULT across the records API.

  Currently to PATCH column id 100 with a new value of "foo", we send:

  {
    "100": "foo"
  }

  How would we PATCH that column with a value of DEFAULT?

• My inclination would be to use a schema like this:

  {
    "100": { "default": true }
  }

  This potential feature and implementation is one reason that in Improve handling of non-Mathesar JSON types #1324 I suggested we add an additional layer of nesting if we do want to send JSON data as raw JSON in the API.