pg hooks for online table

[dimitri]

Adding hooks for PG online table.

• PreOnlineTableOp_hook
• PostOnlineTableOp_hook
• OnlineTableSecurityLabel_hook

The 3 hooks are all used to bypass the ownership check on an online table.

The Pre/Post hooks are used to allow non-online table owner to perform a few DDLs on the table, e.g., CREATE INDEX.

• Pre hook will check the DDL type and whether the referenced relation is an online table. If the conditions are met, it will set the session user as the table owner. The rest of the PG code will work since the session is acting as the table owner now.
• Post hook will restore the current user to the session user.

OnlineTableSecurityLabel_hook is used to allow customers to add who can perform DDL on an online table. By default, the databricks_superuser can. OnlineTableSecurityLabel_hook will check if the session user is one of the roles attached to the online table. If it is, it will allow updating the label to add / remove roles. Otherwise, it will throw the permission error.

Why we add them?

1. These 3 hooks are very specific for online table. We don't intend to use them for other purposes.
2. This is also the reason we are not overloading the public hook also. I need to be very specific on the code point where we want to set as table owner. standard_ProcessUtility contains too many stuff. I don't want to make our change generalize to all PG utilities.