Add support for Alter Table Add/Drop Constraint

[ozgune]

We currently don't propagate Alter Table Add/Drop Constraint commands to related shards. This feature request came up in a customer conversation; and I'm opening this issue to track it.

ALTER TABLE triggers DROP CONSTRAINT IF EXISTS triggers_pkey;
ERROR: alter table command is currently supported
DETAIL: Only ADD|DROP COLUMN, SET|DROP NOT NULL, SET|DROP DEFAULT and TYPE subcommands are supported.

[velioglu]

I have started coding this issue today. I think, it will take 6 days to implement it.

[saicitus]

+1, recently came up in a customer conversation.

In their case, the customer wanted to run ALTER TABLE ADD UNIQUE CONSTRAINT. If we currently don't support this particular syntax (due to checks for having the distribution column as part of the primary key), it could be worth opening a separate issue on this.

As a workaround, we created a UNIQUE INDEX.

[velioglu]

@saicitus. After the PR will be merged, user can add unique constraint with ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE(column_name(s)). Note that column_name(s) must include the distribution column.