[pgAdmin4][Patch]: Fix for Feature #4334, RM #4496 and #4497

Hi Hackers,

I have added the support of Generated Columns (PG/EPAS 12) and also fixed
RM #4496, #4497.

During the implementation of that I have also fixed following issues that I
encountered:

- User should not be able to provide Default value with Identity column.
- 'Drop Cascade' button to delete multiple columns should be disabled.
- Set "Not Null?" switch to 'Yes' when we create Identity column.
- Fix issue when we insert default value for the column and then remove
it. It generates wrong msql.
- Fix issue when we provide the default value of a column and then
select the datatype to serial/bigserial/smallserial, it generates wrong
msql.

Modified/Added regression test cases for adding Identity column and
Generated columns.
Please review it.

--
*Thanks & Regards*
*Akshay Joshi*

*Sr. Software Architect*
*EnterpriseDB Software India Private Limited*
*Mobile: +91 976-788-8246*