[pgAdmin4][Patch]: Feature #4283 pgAdmin4 support for v12

Hi Hackers,

I have started adding support of PostgreSQL v12 in pgAdmin4. Attached is
the patch to fix the following issues:

- RM #4290: "string indices must be integers" displayed for all objects
- RM #4288: View data issue.
- PG v12 removes the ability to create table using WITH OIDS, so I have
modified the code in GUI as well as on backend.
- PG v12 removes obsolete pg_attrdef.adsrc and pg_constraint.consrc
column, so fixed with appropriate functions calls.
- For partitioned tables, since no storage is required for the table
itself. PG v12 throws and error while specifying "TABLESPACE" in create
table syntax for partitioned table, so fix that too.

This is the initial patch to fix the issues that has been raised or I have
found. Will continue to work on adding support for v12. Will check the
release notes.

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*Thanks & Regards*
*Akshay Joshi*

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