service/header/core_listener: Ensure core node connection is not syncing before publishing headers to headersub

[renaynay]

Problem

Old block headers (those older than network head) are getting spammed to headersub.

Solution

In order to prevent the spamming of old block headers to headersub, Bridge nodes should ensure that their running celestia-core node connection is NOT syncing before publishing new ExtendedHeaders to headersub.

This can be done by only launching the listen loop after the core.BlockFetcher is not syncing. A new method can be implemented for BlockFetcher to return whether core connection is syncing or not and the listen loop can wait to launch until it is no longer syncing.