Integrate with Key module for secure storage of OAuth 2.0 credentials

Problem/Motivation

Currently, the feeds_http_oauth module stores OAuth 2.0 credentials (Client ID, Client Secret, Username, and Password) directly within the feed configuration as plain text. While this is functional, it presents several security risks and management challenges:

1. Plaintext Storage: Secrets are stored in the database and exported via configuration as plain text, making them vulnerable if the database or configuration exports are compromised.
2. Hardcoded Secrets: Users are forced to enter sensitive information directly into the UI, which may be visible to other administrators.
3. Lack of Centralization: Updating a secret (e.g., rotating a Client Secret) requires editing every individual feed instance that uses it, rather than updating it in one central location.

Proposed resolution

Integrate the module with the Key module to leverage Drupal's standard for secure credential management.

1. Dependency: Add key as a module dependency in feeds_http_oauth.info.yml.
2. Plugin Refactor: Update HttpOAuthFetcher.php to use Dependency Injection to access the key.repository service.
3. Secure UI: Update the HttpOAuthFetcherFeedForm.php to replace standard text and password fields with the key_select element. This allows users to select a pre-defined Key instead of typing secret values.
4. Runtime Retrieval: Modify the token acquisition logic to fetch the real values from the Key module only at the moment of the request.

Remaining tasks

Review and testing.

User interface changes

The text fields for Client ID, Client Secret, Username, and Password in the OAuth 2.0 settings are replaced with dropdown menus that list available keys from the Key module.