Provide an implementation for avg / abs function with proper type #13049

Describe the use case

Currently avg and abs do not have an implementation in the functions module

Since these are fairly common i thing we could provide an implementation so that the correct type can be inferred.

• avg is numeric (or Double)
• abs instead derives its type from the input

Databases / Backends / Drivers targeted

Postgresql but any db should apply

Example Use

assert func.abs(sa.column('a', sa.Integere).type == sa.Integer()

Additional context

No response