This PR is a WIP from a code standpoint, but am requesting a functional review. Items still needed:

• Cleanup around mapping / migration iteration
• Support deleteByNamespace

Summary

This is an initial pass at adding support to Saved Objects to allow document schema definitions to define a custom index name for something other .kibana

Note: schema prop name of indexPattern is used even though a "pattern" is not yet supported as a future/followup PR should add support for a pattern

API

To make use of this new feature all you must do is define the index in the savedObjectSchemas values of your plugins init like so:

{
    version: '8.2.3',
    uiExports: {
      savedObjectMappings: [
        {
          pluginId: 'testtype',
          properties: {
            testtype: {
              properties: {
                name: { type: 'keyword' },
              },
            },
          },
        },
        {
          pluginId: 'testtype2',
          properties: {
            testtype2: {
              properties: {
                name: { type: 'keyword' },
              },
            },
          },
        },
      ],
      savedObjectSchemas: {
        testtype: {
          indexPattern: 'other-index',
        },
      },
    }

Testing

This PR is small, but affects a LOT of surface area so I am breaking down areas that need special attention. (please comment if I missed something)
While much if not all of this is covered by automated testing, the fact that SOs is critical to Kibana function I think demands a little extra special attention

• Works with spaces (@kobelb)
• Works with feature controls (@kobelb)
• SO API
• SO hidden features
• Migrations console optput