Allow local peer connections on Regtest

[arya2]

Motivation

This will be needed for the RPC tests and to match zcashd's behaviour on Regtest as closely as possible.

Design

On Regtest:

• Return the configured initial testnet peers without loading the peer cache in zebra_network::Config.initial_peers() instead of returning an empty set, and
• Reject incoming connections and avoid attempting outbound connections with peers outside the local host domain.

[mpguerra]

How does this relate to #8361 ?

[arya2]

Fixed by https://github.com/ZcashFoundation/zebra/pull/9580.

How does this relate to https://github.com/ZcashFoundation/zebra/issues/8361 ?

#8361 would be cleanup that also allows for disabling all peers on Regtest, or for disabling all or non-local peers on configured Testnets. It wouldn't change the behaviour much, originally we wanted to get rid of any calls to is_default_testnet() or is_regtest() and replace them with fields.