[Tech debt] SQL enhancements in 2.10

[art-dr]

Estimated at: 11pt

Dev board ref: https://github.com/tarantool/roadmap-internal/issues/100
PM ref: https://www.notion.so/tarantool/Process-Monitor-Project-43e66e6f43c64bbeb4ae6902fc67e315
SME: @ ImeevMA

Tasks

• custom aggregate functions for SQL #2620
• Different sorting order in SELECT statements with LIMIT clause #2621
• SQL: new rules to determine type of result of CASE #3184

[patiencedaur]

SQL is only applicable to single-node Tarantool, which is not the most popular use case.