Implement polykey nodes join command

[aryanjassal]

Description

This PR works alongside MatrixAI/Polykey#924 to allow an agent to join a PKE network. This PR focuses on introducing a nodes join command which allows a node to join the provided network.

Issues Fixed

• Fixes Create the polykey nodes join command #434

Tasks

• 1. Add the polykey nodes join command
• 2. Add tests for the command by simulating private networks

Final checklist

• Domain specific tests
• Full tests
• Updated inline-comment documentation
• Lint fixed
• Squash and rebased
• Sanity check the final build

[aryanjassal]

For testing nodes join, I need to modify the behaviour of DNS resolution to point to a local node. For doing this, the dns.promises.resolveSrv is called to resolve the service _polykey_agent._udp.${hostname} to return a pair of [name, port]. The name is of the format ${nodeId}.${hostname}, so we need to trim the nodeId out to get the hostname from the name.

To test this, I can mock dns.promises.resolveSrv to return the name and port of a local node which I can easily construct and mock. I can reference the tests in polykey as well for things that I am currently unsure about, but this will allow me to easily test network switching from a network to another.