Flutter & cloud_firestore: Adding the function that can get specific fields.

[SittiphanSittisak]

From https://stackoverflow.com/questions/71071246/get-certain-fields-in-document-firestore-flutter

Can you add the function from this answer? (DocumentMask and Projection)
I see many questions about 'How to get the specific field of each document without getting the whole field?'
At the moment, the answer is like this answer in this link.

I think this is good to add these functions for the best security and use less of the internet for the best user experience when the internet is slow.