Fix column ordering for Redshift tables across ADDITIONs

[alexanderdean]

Column order for a com.acme/eg-schema/1-0-3 should match the order of columns if all the individual migration scripts (per #134) were applied.

The rules should be as follows:

• Columns will be alphabetically ordered within an initial release (e.g. 1-0-0) - we needed a way of ordering JSON dictionary fields, which are intrinsically unordered. Alphabetical seems the safest
• New columns in an ADDITION release (e.g. 1-0-1) will be added at the end of the table (so they can be added via ALTER), and alphabetically ordered among themselves

Column order for REVISIONs is out of scope of this ticket - let's focus on ADDITIONs first.

[alexanderdean]

/cc @lesjonesiv, @yalisassoon