Implement API: GET /countries/leaderboard?continent=Asia

[dadiorchen]

To implement an endpoint: Implement API: GET /countries/leaderboard?continent=Asia

• Please write an e2e test to cover it.
• Please follow the guide in the 'readme' to follow our architecture on the server-side.

Note, the leaderboard has been created, but don't support query: continent=name, so the goal of this issue is to filter the leaderboard by continent name

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Some hints:

• Please read our readme for more information/guide/tutorial.
• Here is an engineering book in Greenstand.
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[dadiorchen]

Also, you will find we 'hard code' the data in the API, that's just for that we are able to return the data quickly, so the development experience is good on the client side, please restore the real SQL query and add your code

[dadiorchen]

We will figure out a way to speed up the performance like a SQL cache.

[pierrelstan]

@dadiorchen I will work on this one

[dadiorchen]

Welcome to the GIS world :D