Replace Projection.alias with SubqueryAlias

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
In #2172 we added a new SubqueryAlias operator in the logical plan to wrap aliased table scans. This issue is to apply the same approach to projections and remove the alias field from the Projection struct.

Describe the solution you'd like
As described above.

Describe alternatives you've considered
None

Additional context
None