Move Accumulator trait to physical-expr

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
`Accumulator` is the state of an aggregate function, and has nothing to do with a logical expression.

The current blocker is AggregateUDF which mixes both the function signature and the function implementation. AggregateUDF exists in `datafusion-expr` because of its signature parts.

**Describe the solution you'd like**
1. Move Accumulator trait into `datafusion-physical-expr`.
2. Split AggregateUDF into parts, one remains in `datafusion-expr` that represents AggregateUDF's signature, and the other in `datafusion-physical-expr` for its implementation.

**Describe alternatives you've considered**
Don't move.

Start by locating the Accumulator trait and AggregateUDF definitions in the datafusion-expr and datafusion-physical-expr crates, then trace their dependencies and current uses. The work is done when Accumulator is owned by physical-expr and AggregateUDF is split between signature and implementation parts without breaking the existing crate interfaces.

Written by the indexing model from the issue text.