Include username in log entry if it is available

What does this MR do and why?

Partially resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/531

Update Telesign callback request logging to include the username associated with the found (if any) Users::PhoneNumberValidation record. This will make it easier to search for Telesign callback request log entries for a given user.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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Before	After

How to set up and validate locally

Note: Use the values below as-is for easier testing

1. Setup Telesign

   $ rails c
   > ApplicationSetting.first.update(telesign_customer_xid: 'test_cust_id', telesign_api_key: 'test_api_key')

2. Create a test Users::PhoneNumberValidation record

   $ rails c
   > user = FactoryBot.create(:user, email: 'feb2422@ex.com', username: 'feb2422')
   > FactoryBot.create(:phone_number_validation, telesign_reference_xid: '2557312299CC1304904080F4BE17BFB4', user: user)

3. Follow the application logs

   $ tail -f log/application_json.log

4. Send a test callback request

   $ curl -X POST http://localhost:3000/-/phone_verification/telesign_callback -H 'Authorization: TSA test_cust_id:bqeIi0UHnk1inbM3rNGN1JoaPx8YVo7tNU6eOxCFsWY=' -H 'Content-Type: application/json' -d '{ "status": { "updated_on": "2016-07-08T20:52:46.417428Z", "code": 200, "description": "Sent" }, "errors": {}, "reference_id": "2557312299CC1304904080F4BE17BFB4" }'

5. Validate that you get a log entry that looks like

{"severity":"INFO","time":"2024-02-02T08:51:41.207Z","correlation_id":"01HNMG8BN13S9HWYH8J1V33GET",...,"class":"Telesign::TransactionCallback","username":"feb2422","event":"Telesign transaction status update",...}