Add `has_XXX_summaries` entity attributes

[mxr]

These attributes can be used to determine whether or not various summaries exist on the state. That way I can easily determine whether or not to show/hide cards for the summaries. For example I could write a dashboard card such as:

type: entity-filter
entities:
  - type: attribute
    entity: sensor.mta_subway_l
    attribute: delay_summaries
    name: Delays
    state_filter:
      - operator: '=='
        value: true
        attribute: has_delay_summaries