feat: implement oauth for public apis

[tiwarishubham635]

Fixes

This PR adds support for accessing Twilio APIs using OAuth2 as well. It adds the following classes:

1. Token Managers to fetch access token
2. AuthStrategy to use tokens and create authString
3. Credential Providers to choose different auth strategy, be it - Basic Auth, OAuth or NoAuth (Limited use)
4. Modifications to Client to accomodate new changes without breaking previous flow

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
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• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

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