feat: Oauth design

[manisha1997]

Feature

This PR adds Oauth to twilio-go.
Changes added:

1. Customers will now use the ClientCredentialProvider object and pass Client Credential parameters.
2. The struct Client has Oauth object which contains the following:
   a. The IamService Object used --> This will act as an interface for any IAM service we plan to access
   b. The Credentials details
3. Added test cases

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
• I have updated my branch with the main branch
• 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

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.

[Copilot]

Pull Request Overview

This pull request introduces OAuth support by adding a new token authentication mechanism and updating corresponding API service and client methods. Key changes include:

• A new package module (oauth.go) that implements token fetching and expiration using JWT.
• Updates to REST API documentation, including new endpoints and model definitions for OAuth tokens.
• Enhancements in the client and base client to support OAuth integration alongside basic authentication.

Reviewed Changes

Copilot reviewed 62 out of 62 changed files in this pull request and generated no comments.

File	Description
oauth.go	Adds TokenAuth and APIOAuth implementations for OAuth token handling.
oauth_test.go	Introduces tests for token fetching and expiration checking.
rest/iam/v1/, rest/preview_iam/v1/	Updates and adds documentation and API service methods for OAuth.
client/client.go and client/base_client.go	Enhances client functionality to support OAuth with new getters/setters.

Comments suppressed due to low confidence (2)

oauth.go:71

• Setting the OAuth interface to nil before creating a token might lead to unintended behavior. Please review this line to ensure that the OAuth object is managed correctly.

a.iamService.RequestHandler().Client.SetOauth(nil)

client/client.go:235

• [nitpick] Consider renaming 'SetOauth' to 'SetOAuth' for consistency with the 'GetOAuth' method and to improve readability.

func (c *Client) SetOauth(oauth OAuth) {

[kevinburkesegment]

OK, we're getting closer. Still hoping for a test on expirations with UTC vs. other time zone.

[manisha1997]

OK, we're getting closer. Still hoping for a test on expirations with UTC vs. other time zone.

I have now coverted all date to UTC. Do we still need these tests?