feat(network): Configurable external address for Version network messages

[oxarbitrage]

Motivation

We want to add an optional configurable external address to advertise in Version messages.

Close #1890

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Solution

I think this is the simplest change we can apply to support this feature. Having multiple external addresses and/or auto detection can be done in the context of #1893

Testing

Some basic test was added.

Review

Anyone can review.

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Follow Up Work

[arya2]

Looks good! Thank you. I left a few comments, none of them are blockers, feel free to resolve any or all of them without making changes.

It may be nice to add the new field to a test config in zebrad::tests::configs as well.