Send globally unique instance and user IDs to AI gateway

What does this MR do and why?

Send hashed instance and user IDs to AI gateway for tracking purposes. We want these IDs to be:

• Globally unique
• As stable as can be
• Work for both SM and SaaS
• Hard to trace back to original user

The solution we landed on is:

• Global instance ID: CurrentSetting.uuid, which is already used for Service Ping, and which is populated as part of ApplicationSettings so should always exist.
• Global user ID: Base64(SHA256(uuid, salt, user.id)); since the numeric user ID is locally unique to an instance and the UUID is globally unique, they will hash to a unique global UID.

We send these in two new custom headers:

• X-Gitlab-Instance-Id
• X-Gitlab-Global-User-Id

The gateway can then log these in access logs and/or track them with Snowplow (see gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#192 (closed)).

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #416621