* feat: UDAF implementation backed by DataFusion.
Directly Transplant DataFusion's UDAF related structs, traits and functions, like `AggregateUDF`, `Accumulator` or `create_udaf` etc.
Implement median UDAF on top of it and used in unit testing.
Refs: #61
* feat: UDAF made generically
Refs: #61
* fix: cargo fmt
* fix: use prelude
* fix: uniform the name
* fix: move maybe commonly used functions together
* fix: make comments more clear
* fix: resolve conversations in CR
* fix: store input types in AccumulatorCreator, and use ScalarVector's iterator
* feat: introducing List value and List datatype
* refactor: use ArcSwap instead of Mutext
* refactor: shorten some namings
* refactor: move median UDAF out of tests
* refactor: rename
* feat: aggregate function registry
* fix: make `Value` satisfy ordering again
* fix: clippy warnings
* doc: add "how to write aggregate function"
* fix: address PR comments
* fix: trying to get rid of unwraps
Co-authored-by: luofucong <luofucong@greptime.com>
