feat: Add showdown field to pokemon api resource

[FallenDeity]

This pr includes a new field in the pokemon endpoint which is the showdown field containing animated gif sprites for each pokemon.

This is a followup to this pr and #977 which was added a while back and allows these resources to be accessed from the api.

"showdown":{
            "front_default":"https://raw.githubusercontent.com/PokeAPI/sprites/master/sprites/pokemon/other/showdown/1.gif",
            "front_shiny":"https://raw.githubusercontent.com/PokeAPI/sprites/master/sprites/pokemon/other/showdown/shiny/1.gif",
            "front_female":null,
            "front_shiny_female":null,
            "back_default":"https://raw.githubusercontent.com/PokeAPI/sprites/master/sprites/pokemon/other/showdown/back/1.gif",
            "back_shiny":"https://raw.githubusercontent.com/PokeAPI/sprites/master/sprites/pokemon/other/showdown/back/shiny/1.gif",
            "back_female":null,
            "back_shiny_female":null
         }

Above is the sample field after update and here is the data as a whole data.json

This pr also includes tests for the new addition here

self.assertEqual(
            sprites_data["other"]["showdown"]["front_default"],
            response_sprites_data["other"]["showdown"]["front_default"],
        )
        self.assertEqual(
            sprites_data["other"]["showdown"]["back_default"],
            response_sprites_data["other"]["showdown"]["back_default"],
        )

[MakashiDev]

Looks good hope to see it integrated soon!

[pokeapi-machine-user]

A PokeAPI/api-data refresh has started. In ~45 minutes the staging branch of PokeAPI/api-data will be pushed with the new generated data.

The staging branch will be deployed in our staging environment and the entire API will be ready to review.

A Pull Request (master<-staging) will be also created at PokeAPI/api-data and assigned to the PokeAPI Core team to be reviewed. If approved and merged new data will soon be available worldwide at pokeapi.co.

[pokeapi-machine-user]

The updater script has finished its job and has now opened a Pull Request towards PokeAPI/api-data with the updated data.

The Pull Request can be seen deployed in our staging environment when CircleCI deploy will be finished (check the start time of the last build).