Implement `display_temperature` for FGL devices.

[gyro-labs]

FGL devices report their temperature in a weird unit (centigrades from -50C), so I added a parser metadata item for fields that need this.

Also added some debugging logs to better understand the flow of information in this application.

[claesmathias]

I received display temperature 748 instead of 25 degrees Celsius for my Fujitsu FGLair device.

[vyruz1986]

@deiger Any chance you could take a look at this PR and either merge it or provide feedback? We have #240 open since july and it looks like the solution is here just waiting to get merged.

As I mentioned in the issue, I get that you're probably busy, but thanks in advance for trying to give this some priority :)

[xury77]

@vyruz1986 I merged those PR in my cloned repository and create multiarch docker images. I use with Fujitsu FGlair. Now current temperature is displayed properly using MQTT message, but still has null value in entity attributes. I can't fix it because my knowledge of Python is very poor. You can try it using my repo.