Allow setting a custom placement policy per table through yb-admin_cl…

[deeps1991]

…i that overrides the cluster placement policy.

[vvkgopalan]

One case to account for is when a table is colocated / part of a tablegroup, in which case setting a custom placement policy for that table should be dis-allowed. This is because colocated tables / tables within a tablegroup are placed on a singular parent tablet. For reference on implementation, see 6ef74d8 and 9fa72cd.

In the case of tablegroups, it may make sense to have a separate command to enable setting this placement policy per-tablegroup (so that all the tables within the tablegroup also obey it).

[rahuldesirazu]

This is looking awesome, great work!! A few high level comments/questions:

1. Will a user be able to create a table with a custom placement policy without having to modify it after the fact with yb-admin?
2. Currently this feature may not work if we want a table level policy with affinitized leaders or read replicas. Wanted to clarify the scope of this code change and check whether you want to address that here or in a follow up PR.
3. Since the LB works in sub-clusters defined by placement_uuid, we'll want to make sure the live/primary table placement_uuid always matches the live/primary cluster placement_uuid.

[rahuldesirazu]

We can probably remove optional_uuid, since I think we'll always want the live uuid of the table placement to match the uuid of the cluster level placement.

[deeps1991]

I left it here to be future proof, but added a check to ensure that the two uuids match. Let me know if that is okay, or I can remove this in the next patch.

[deeps1991]

Thanks for the review Vivek and Rahul! I have addressed all the comments in the last commit.
@rahuldesirazu I've implemented the change based on our offline slack discussion - and implemented checks to verify that
a) We set the custom table placement info only for a cluster with just live placement info set.
b) The placement uuid for the table matches the placement uuid of the cluster
c) We only set live placement info for a table.

Let me know if my understanding went incorrect somewhere, and I can fix those in a follow up patch.

[rahuldesirazu]

Everything looking really solid! Just a small test case to add and a nit fix.