Add custom state template and custom service call to button

[IzeQube]

Adds the option to define a few extras for advanced user via an extra key extras in the YAML configuration.

1. It's possible to define a template for the displayed button state:

- type: button
id: 1063494948270
entity_id: switch.switch_buero_pc_sonos_strip
name: O's PC, Sonos, Strip
extras:
  state_template: >
    {% if(is_state('switch.switch_buero_pc_sonos_strip', 'on'))
    %}
    {{ "An • " + states('sensor.switch_buero_pc_sonos_strip_power', with_unit=True) }}
  {% elif(is_state('switch.switch_buero_pc_sonos_strip',
  'off')) %}
    Aus
  {% else %}
    {{ states('switch.switch_buero_pc_sonos_strip') }}
  {% endif %}

2. It's possible to define a custom service call. If a entity_id exists in service_data it is used for the "waiting animation" of the button icon:

- type: button
id: 1113219359563
entity_id: switch.switch_buero_monitore_co
name: J's PC, Monitore
more_info: false
icon: mdi:power-socket-de
extras:
  service_domain: light
  service_service: turn_on
  service_data:
    entity_id: light.hue_buero

Fixes #209

[matt8707]

Thanks, for the PR! I added this in ui so I'll close this one