Validate files containing descriptions of AWS services

[k-a-il]

The goal of this PR is to add a reusable CI/CD pipeline to validate files containing descriptions of AWS services (aka. feature catalog files). This reusable workflow will be used in localstack/localstack and localstack/localstack-ext repositories as in this example.

What is part of this PR?
A CI/CD pipeline that performs the following tasks:

• Check that features.yml file exists
• Validate changed features.yml files against JSON schema

Example of testing done in a forked repo:

• feature file is missing but provider.py was modified: https://github.com/k-a-il/localstack/pull/7/checks?check_run_id=39717634300
• feature file was added but it does not follow JSON schema: https://github.com/k-a-il/localstack/pull/6/checks?sha=bbe2257d623def55b97eb218f620e5f1ec611455
• feature file validated against schema: https://github.com/k-a-il/localstack/pull/6/checks?check_run_id=39717515319

[silv-io]

LGTM! Just some questions/suggestions :)

[sannya-singal]

Nice work, thanks for adding this 🚀 I have some comments and a question, rest LGTM.