[Utility] Relays should be signed by the servicer

[adshmh]

Objective

The servicer needs to sign all relays. This requires passing a private key to the servicer, as well as the logic to sign all the serviced relays.

Origin Document

Part of work on #754

Goals

• Every serviced relay is signed using servicer's private key.

Deliverable

• Updated configuration and other support required for providing a private key to the servicer
• Updated servicer logic to sign every relay
• Updated documentation for configuring the servicer

Non-goals / Non-deliverables

• Support for ring signatures or delegated relays

General issue deliverables

• Update any relevant local/global README(s)
• Update relevant source code tree explanations
• Add or update any relevant or supporting mermaid diagrams

Testing Methodology

• Task specific tests or benchmarks: make ...
• New tests or benchmarks: make ...
• All tests: make test_all
• LocalNet: verify a LocalNet is still functioning correctly by following the instructions at docs/development/README.md
• k8s LocalNet: verify a k8s LocalNet is still functioning correctly by following the instructions here

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Creator: @adshmh
Co-Owners: @Olshansk

[Olshansk]

I've made it explicit that this is a non-goal: - Support for ring signatures or delegated relays