add(network): Adds a network magic field to testnet::Parameters and the config

[arya2]

Motivation

We want to allow for configuring the network magic on custom testnets.

Part of #7845.
Depends-On: #8518.

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Solution

• Moves network Magic type from zebra-network to zebra-chain
• Adds a network_magic field on testnet::Parameters and uses it in the network codec
• Reads a network magic from the config

Testing

• Adds a stored test config
• Updates a Magic debug impl test
• Adds a test to check that the codec returns an error when messages have unexpected network magics

We could also add a test for the constraints in testnet::ParametersBuilder::with_network_magic() (can't be the network magic of a default network) if we want.

Review

Anyone can review.

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Follow Up Work

Accept peer IPs in the config as initial peers.

[oxarbitrage]

Looks good. Thanks.