feat(transformation): design specs for supporting dp sql queries as transformation in dcr

[qascade]

Description

SQL queries are much more user-friendly and readable than any other general-purpose languages like Python or Go. We want to support such SQL queries as transformation if possible, but the catch is that the query result should be differentially private. We may have to look into different strategies on how we can generate an execution plan based on query input that makes the SQL output dp. We would also want the query execution to be done inside a tee.

Some high level strategies may include:

1. Take any existing DB and modify its query language and execution plan that uses google's dp definition
2. Write an OLAP db from scratch and make sure the above conditions are met.
3. Write an interpreter that generates confidential go-apps after reading a SQL query. (In this we will be only able to support a limited set of queries and may have to go from query by query)

Resources:

1. https://dpfordb.github.io/dpfordb.pdf
2. https://github.com/google/differential-privacy

[qascade]

Implemenation of this spec is to be done in #59