Introduces field interface

  getName() //unique name of the field
  isValid() //whether it is usable (e.g. partial field configs can arise when editing).
  async getLabel() //display of the field
  async createTooltipProperty()//create corresponding tooltip entry

Introduces 4 field types (x-pack/legacy/plugins/maps/public/layers/fields)

• EMSRegionField: fields from EMS
• ESAggField: fields that wrap results of aggs (e.g. geogrid-sources, or term-join metrics)
• ESDocField: fields from ES-index pattern
• KibanaRegionField: fields from configured geojson in the .yml

Introduce fields as top-level concept.

Fields are scattered and inconsistent throughout the code-base right now, wrt. display, config, and typing.

Fields should be used consistently for:

• tooltips
• top hits sort and split field editor
• joins
  -- left field definitions
  -- right field definitions
• dynamic styling properties
• agg metrics
  -- for grid source
  -- for pewpew source

Fields are source-dependent

• EMS
• ES fields (agg results and docs)
• Kibana layer configs

Later, fields should also be used for

• automagic suggestion of styling params (e.g. categorical vs ordinal styling)
• preloading of data-ranges (e.g. to determine intelligent breaks in color ramp)
• identifying whether field is id-field or display-field (this is EMS top-level metadata, unused by Maps)