change(network): Allow custom testnets to make peer connections and configure more parameters

[arya2]

Motivation

We want to support configuring more custom testnet parameters and to allow Zebra to make peer connections on custom testnets.

Depends-On: #8524

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Solution

• Replace check for is_default_testnet() in initial_peer_hostnames() with !is_regtest()
• Add the remaining configurable parameters in testnet::Parameters to config deserialization

Related changes:

• Return early with an empty set from initial_peers() when network.is_regtest() (Zebra shouldn't ever add any peers to the peer cache on Regtest, because it has no initial peers, but it the file could be edited outside Zebra)

Testing

Updated the latest stored test config

Review

Anyone can review.

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The base branch was changed.

[oxarbitrage]

Using the main branch i cherry picked the relevant commits and force pushed to fix the conflicts this PR had.