support data transformation on the query result before visualize

[shawncao]

Sometimes, user have defined metrics on the query result itself. For example

1. new metric column per row = col(A) / col(B)
2. further aggregate across different rows: (A B 1) (A C 2) => (A B/C=0.5)

No matter what - we will pass the raw JSON blob to user and let it transform uses its own transform lambda, new schema will be discovered in the transformed result.

[shawncao]

solved by SDK to support pivot and map functions.