feat: Prototype PR for fine grained access control

[hemanshv]

Hi @jskeet @amanda-tarafa This is a prototype PR to demonstrate high level approach for spanner fine grained access control. It is still very much a work in progress, and formatting, docs are not done yet. Also, there is an issue with the whitespace/line-ending and file-encoding in my visual studio, so there are few whitespace changes which are not intended. I am trying to fix it. Please have a look at the approach and provide your valuable feedback. Thank you