Support registering agent with extra telemetry data

This change set adds to support for agents to register themselves with
extra telemetry data consisting of key-value pairs. They can be
arbitrary from a KAS and agentk perspective, but they will be checked in
Rails. Currently, there are "arbitrary" limits for how many extra data
can be sent and how long they can be.

Refs https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/issues/594