[#38] Add schema validation for functions, script function bug fixed

[jekalmin]

Objective

• Add schema validation for functions
• If function is not valid, throw an error before calling API.

Bug Fix

• script function should support all actions like if-then statement.

  - spec:
      ...
    function:
      type: script
      sequence:
        - if:
            - condition: state
              entity_id: ...
              state: "..."
          then:
            - service: media_player.select_source
              data: ...