Added Oauth2 token generation

[abhinavs1920]

PR Description

This pull request implements the core functionalities of the OAuth2 feature, specifically focusing on configuration saving and changes. The implementation utilizes the official Dart OAuth2 package and uses GitHub as the OAuth provider for illustration purposes. The Authorization Code flow type is currently implemented.

As per the last PR #524, I have made changes to my code.

I will be very happy if you would guide me for the improvements I need to do.

Related Issues

• Closes Implement OAuth 2.0 authentication #481

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with the project main branch before making this PR
• I am using the latest Flutter stable branch (run flutter upgrade and verify)
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

We encourage you to add relevant test cases.

• Yes
• No, and this is why: please replace this line with details on why tests have not been included

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux

[abhinavs1920]

@ashitaprasad please review my PR. Let me know the changes required.

[ashitaprasad]

The OAuth implementation is currently at a global app level which is not the right approach. Also, the design system is not being followed.
It is recommended you learn more about API Auth for API testing and start with a simpler form of authentication first.