Support adding unique constraints through ALTER TABLE

[fulghum]

Doltgres supports adding a unique constraint via a primary key, but returns the following error for a non-primary key uniqueness constraint: Only PRIMARY KEY constraints are supported currently

The spot where we're missing logic is:

doltgresql/server/ast/constraint_table_def.go

Line 63 in a2b63ab

return nil, fmt.Errorf("Only PRIMARY KEY constraints are supported currently")

Discovered through HarukaMa data dump loading.