[price-service] add /get_vaa endpoint that serves cache and db vaas

[cctdaniel]

generally:

• store the VAAs in an object with the price feed id as the key and the value is an array of objects that has a publishTime and vaa (VAAs are stored in an ascending order based on publishTime)
• because VAAs are stored in an ascending order based on publishTime, we can run binary search on the array to find the publishTime greater than or equal to the target on O(logn) time
• the endpoint first hits the cache and tries to find the VAA, if it doesn't exists it hits the DB
• every minute, the cache will be cleaned up, and publishTime that has expired will be removed from the cache

other considerations:

• balanced bst (not trivial + since the array is already sorted we can just use binary search)

[ali-behjati]

I don't think injecting pyth writer logic be good here. The check seems unnecessary to me.

[ali-behjati]

🚢 it ser