patch: create seperate serializers for sites and observations

[tinashechiraya]

The existing serializers return combined data for sites and observations as this is the expected result on the frontend however for this API the data requires a better structure which is why i have created these additional serialzers so that the output structure matches the one requested by the client

example usage: '/sites-with-observations/?start_date=2023-03-13'

[lgkgh]

Jenkins Build Details:

Build status: SUCCESS Build duration: 16 min