Adding local tables to metadata: Limitations & TO-DO's

## Limitations:

#### Existing limitations affecting citus local tables as well as other citus tables:
* Several limitations for DDL commands #4812
* Not supporting `INSERT INTO citus_local_table` with subqueries.
* ~~Not supporting IDENTITY columns~~ Now supported #4660
* Not allowing citus local tables to be involved in any multi-level partitioning.
* Not allowing citus local tables to be involved in any inheritance relationship with other tables.

#### Limitations specific to citus local tables:
* Not supporting foreign keys from reference tables to citus local tables with `CASCADE`, `SET NULL` & `SET DEFAULT` behaviors (`RESTRICT` & `NO ACTION` behaviors are still allowed)
* Not allowing `UPDATE/DELETE`s involving postgres local and citus local tables
* Queries from MX nodes on citus local tables having triggers might fail due to 2PC within xact blocks https://github.com/citusdata/citus/issues/4148
* Not allowing `UPDATE/DELETE` of reference tables selecting from citus local tables (other way around is just OK).

#### Limitations that we **might not** think to resolve:
* ~Not supporting creating foreign keys between citus local tables and postgres tables
Solving this problem is equivalent to bringing foreign key support between postgres local tables and reference tables and that was the main point that leaded us to bring citus local tables :)~

## Considerations & further improvements on citus local tabes:
* We don't allow creating citus local tables in MX worker nodes or moving citus local tables from coordinator to other MX worker nodes.
* Related with above item, `ColocationId` for citus local tables is set to `INVALID_COLOCATION_ID`.
We might think of setting common `colocationId`s for citus local tables thinking of them as grouped according to the nodes that their shard live after enabling above item.

## Some other TO-DO's:
* ~We need to fix citus local table creation from tables with GENERATED STORED columns (#4151)~
* ~We need to adjust pre-existing views on citus local tables after citus local table creation so that pre-existing views point to the shell relation, not to the shard relation. https://github.com/citusdata/citus/issues/4147~
* Enable volatile functions in DML's:
```sql
update citus_local_table set a = random()::int;
DEBUG: functions used in UPDATE queries on distributed tables must not be VOLATILE
ERROR: functions used in UPDATE queries on distributed tables must not be VOLATILE
```
* Support deferred fast-path planning to allow Citus local tables rely on caching.
* Support this using ctes's wrapping distributed tables to modify postgres local tables:
```sql
WITH cte AS (SELECT * FROM distributed_table) update postgres_local_table SET a = cte.a FROM cte;
```
(Hint: `DeferErrorIfUnsupportedModifyQueryWithPostgresLocalTable`)
* Test function call delegation that was implemented for reference tables. Should it work with Citus local tables ? https://github.com/citusdata/citus/pull/4143#discussion_r484906356
* Not automatically remove user-added local tables from metadata (#4691)
* Not undistribute local table added to metadata when creating reference or distributed table (#4692)

Start by reviewing the limitations and TODO list in this issue, then inspect the named entry point DeferErrorIfUnsupportedModifyQueryWithPostgresLocalTable and related issues #4148, #4691, and #4692. A completed contribution would need to select one specific limitation or TODO, define its supported behavior, and add or update the relevant tests, but this issue does not identify those test locations.

Written by the indexing model from the issue text.