Add example for aws rds postgres integration with Kanister

[PrasadG193]

Change Overview

This PR adds an example to demonstrate Kanister integration with AWS RDS PostgreSQL engine

Pull request type

Please check the type of change your PR introduces:

• Work in Progress
• Refactoring (no functional changes, no api changes)
• Trivial/Minor
• Bugfix
• Feature
• Documentation

Issues

• Sample blueprint to snapshot/restore AWS RDS database instances #329

Test Plan

• Manual
• Unit test
• E2E

[tdmanv]

Overall looks good, but please fix the licensing. We should remove the apache license and add our copyright to every file and append to the kubernetes copyrights.

[vkamra]

@PrasadG193 @tdmanv - I'm wondering if we can simplify this top level README. The only requirement for someone to test this out should be:

1. Create a new RDS instance or have instanceID/host info available for an existing instance
2. Create a ConfigMap with this info
3. Create a Blueprint
4. Run backup/restore commands

The pgtest app is useful for testing but complicates this example.

[PrasadG193]

Won't that confuse users? They may not get the context behind why do they need configmap and which configmap to use/update.
In current PR we can add a note suggesting instead of pgtest app, they can use their own application

[vkamra]

Users in this example likely already have an application that is using an RDS DB and pgtest on the top level will be confusing.

What will be most useful for them will be to know how they can use this example blueprint with their DB instance.

I think the pgtest example is useful but perhaps this detailed README.md is in a sub-folder and the top level is very minimal

[PrasadG193]

@vkamra @tdmanv I have updated the README as per your suggestions.