createAggConfig should allow order-specific additions

[joshuarrrr]

Is your feature request related to a problem? Please describe.

When creating a new AggConfig, the default behavior is to also add the newly created AggConfig to the existing list of aggConfigs:

OpenSearch-Dashboards/src/plugins/data/common/search/aggs/agg_configs.ts

Line 149 in bfc59d4

this.aggs.push(aggConfig);

However, order matters when specifying aggregations, and the aggregation Schema supports the property mustBeFirst, which is used by the pie chart to ensure that slices are calculated accurately. The simplest, and likely best, fix is to update createAggConfig to take an additional parameter that allows the new AggConfig to be added to the beginning of the aggConfigs array rather than the end.

Describe the solution you'd like

Maintain existing behavior of createAggConfig, but add a new parameter that can switch the insertion point from the end of the array to the beginning.

Describe alternatives you've considered

The existing logic to enforce mustBeFirst constraints is in the vis_default_editor plugin. But for adding pie charts to VisBuilder, we'd have to recreate this logic, since VisBuilder doesn't use the vis_default_editor. In addition, we'd just like to always order the aggs correctly, rather than displaying and error message and forcing the user to reorder (as the current Pie chart visualization does)

Doing so downstream of the agg service means separating the creation of the new AggConfig from the process of adding it to the existing aggConfigs, and ends up with lots of duplication of the order checking logic and direct manipulation of the aggConfigs array. For example, one implementation might look something like this:

       const aggConfig = aggConfigs?.createAggConfig(
        {
          type: allowedAggTypes[0] || validAggTypes[0],
          schema: schema.name,
          params: {
            field: fieldName,
          },
        },
        {
          addToAggConfigs: false,
        }
      );

      const newAggs = schema.mustBeFirst ? [aggConfig, ...aggs] : [...aggs, aggConfig];
      const newAggConfigs = aggService.createAggConfigs(indexPattern, cloneDeep(newAggs));

Conceptually, the agg service should provide the right methods and tooling to add, remove, and reorder agg configs without requiring lots of manual array manipulation and cloning.

Additional context

Blocks #1619