refactor(sql): extract aggregate handling out into common utility class

[jcrist]

This is an initial refactor moving towards #9170.

Previously every backend implemented their own _aggregate function - many of them copy-pasted (with slight variations) of each other.

To add a new order_by kwarg to _aggregate would require editing all 16 copies of this function, which would be annoying. This PR is an attempt to centralize their implementations into a common Gen class. The class takes a config flag to handle the common cases, the uncommon cases are then handled by backend-specific subclasses.

This also extracts the .agg attribute out to be a class variable (err, descriptor) rather than an instance variable.

An alternative implementation would be adding boolean flags directly to the SQLGlotCompiler class, but IIRC we intentionally moved away from those in the SQLAlchemy -> SQLGlot refactor. Could go either way here.

[jcrist]

An alternative implementation would be adding boolean flags directly to the SQLGlotCompiler class

This would be simpler/less magical and would be my slight preference, went with the current extra level of abstraction since I thought I remembered someone else being against flags on the compiler.

[gforsyth]

This looks good to me -- I also have vague memories of someone having issues with adding flags on the compiler class, but I can't remember any specifics.

I suppose we can always revisit and simplify this implementation if we decide that we want to make the change, but this is a clear win over our existing scattered implementations.