fix: Update config.go and framework_config.go to use transport.Creds for all cases

[modular-magician]

This PR fixes the issue in #14411 by ensuring we use transport.Creds for initializing the credentials/tokenSource in all applicable code-path. Using transport.Creds ensures that the underlying OAuth2 HTTPClient used for token exchange is properly configured for mTLS - the current version of the code in terraform bypasses this logic when calling the oauth2 google package APIs directly.

What is not included in this PR:

1. Removing the custom tokenSource override logic - this is because Terraform persists the custom tokenSource for use in other locations such as for initializing big table grpc client as well as populating data source config for GKE clusters. The transport API for creating HTTPClient does not export the underlying tokenSource for re-use. (This support may be added in a future version of the API, which is currently being re-designed.)
2. Simplifying the duplicated ADC logic in terraform. We will revisit this in the future.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

Update config.go and framework_config.go to use transport.Creds for all cases

Derived from GoogleCloudPlatform/magic-modules#7875

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.