[Cleanup] Refactor LinodeIdentityServer for uniformity

[komer3]

This PR renames LinodeIdentityServer to IdentityServer and adjusts its fields to align with the naming conventions used in ControllerServer. Additionally, NewIdentityServer() has been moved from driver.go to identityserver.go to improve code organization.

These changes enhance code uniformity and maintainability across the codebase

General:

• [ ] Have you removed all sensitive information, including but not limited to access keys and passwords?
• [ ] Have you checked to ensure there aren't other open or closed Pull Requests for the same bug/feature/question?

Pull Request Guidelines:

1. [ ] Does your submission pass tests?
2. [ ] Have you added tests?
3. [ ] Are you addressing a single feature in this PR?
4. [ ] Are your commits atomic, addressing one change per commit?
5. [ ] Are you following the conventions of the language?
6. [ ] Have you saved your large formatting changes for a different PR, so we can focus on your work?
7. [ ] Have you explained your rationale for why this feature is needed?
8. [ ] Have you linked your PR to an open issue

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 38.88889% with 11 lines in your changes missing coverage. Please review.

Project coverage is 37.00%. Comparing base (b45e75c) to head (95f78f8). Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/driver/identityserver.go	40.00%	9 Missing :warning:
internal/driver/driver.go	33.33%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #211 +/- ## ========================================== + Coverage 36.94% 37.00% +0.06% ========================================== Files 23 23 Lines 1746 1751 +5 ========================================== + Hits 645 648 +3 - Misses 1067 1068 +1 - Partials 34 35 +1 ```

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