feat(api-gateway): add user blacklist feature

[deshmukhmayur]

Explain the feature/fix

• Introduced a user blacklist functionality that loads blocked user identifiers from a specified file at startup.
• Added support for a new environment variable BLACKLIST_FILE_PATH to configure the blacklist file location.
• Implemented related functions for managing the blacklist, including loading from file and checking user claims against the blacklist.
• Updated README with instructions on using the blacklist feature and provided an example blacklist file.
• Enhanced test coverage for blacklist functionality.

Assisted-By: Cursor cursoragent@cursor.com

Does this PR introduce a breaking change

No.

Screenshots

N/A

Ready-for-merge Checklist

• Expected files: all files in this pull request are related to one feature request or issue (no stragglers)?
• Does the change have appropriate unit tests?
• Did tests pass?
• Did you update or add any necessary documentation (README.md, WHY.md, etc.)?
• Was this feature demoed and the design review approved?

[yashoswalyo]

LGTM