Fix timestamp values in Arkose Data Exchange payload

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/831+

This MR is an outcome of the recent 2024-07-30: Users cannot complete OAuth sign up (gitlab-com/gl-infra/production#18341 - closed) incident.

It fixes $.timestamp and $.api_source_validation.timestamp values in the Arkose Data Exchange payload which contained incorrect values as reported by Arkose

$.timestamp and $.api_source_validation.timestamp should be a string and an integer, respectively, specifying the milliseconds (currently seconds) elapsed since the UNIX epoch as documented in https://support.arkoselabs.com/hc/en-us/articles/4410529474323-Data-Exchange-Enhanced-Detection-and-API-Source-Validation#DataExchange:EnhancedDetectionandAPISourceValidation-APISourceValidation.

This MR also adds a new ops type feature flag (named arkose_labs_data_exchange) that is enabled by default to provide a way to disable the Arkose Data Exchange feature in case something goes wrong.

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