Job Router client - WebSocket support

What does this MR do?

This is a follow up for Job Router (!5945 - merged) that adds WebSocket connectivity support.

Why was this MR needed?

What's the best way to test this MR?

I tested this with GDK + Runner locally.

What are the relevant issue numbers?

Relates to Implement Runner job proxying (gitlab-org/cluster-integration/gitlab-agent#771 - closed).

Merge request reports

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