[BB-4023] Create custom eCommerce client and automate credential sharing

Description

This PR fixes the issue reported in https://github.com/open-craft/microsite-generator/issues/2

Supporting information

https://tasks.opencraft.com/browse/BB-4023

Testing instructions

1. Pull this PR to src directory of your local devstack
2. Copy config/sample.yaml to config/config.yaml
3. To generate & test LMS configurations -
   • Login to LMS shell - make lms-shell
   • Go to microsite directory - cd /edx/src/microsite-generator
   • Run - make dev.run-lms
   • Check if a new OAuth client named custom-sites-ecommerce-sso has been created from LMS admin.
   • Check if there is a config/_generated.yaml file containing credential info.
4. To generate & test eCommerce configurations -
   • Login to eCommerce shell - make ecommerce-shell
   • Go to microsite directory - cd /edx/src/microsite-generator
   • Run - make dev.run-ecommerce
   • Check if SiteConfiguration has correct OAuth credentials from eCommerce admin.

Deadline

None

Reviewers

• @lgp171188
• @farhaanbukhsh