config: allow reading externalIPs from files

[martinohansen]

Tor hidden services dumps the onion hostname into a file on disk. If Tor and btcd is started together -- for example in a sidecar pattern -- the onion address is not known before executions.

This allows the user to provide --externalip a path to a file containing any hostname, for example the onion address as dumped by the Tor service.

[coveralls]

Pull Request Test Coverage Report for Build 2511340025

• 0 of 21 (0.0%) changed or added relevant lines in 1 file are covered.
• 6 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.03%) to 55.006%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
server.go	0	21	0.0%
<!--	Total:	0	21

Files with Coverage Reduction	New Missed Lines	%
btcutil/gcs/gcs.go	1	81.25%
server.go	1	0%
connmgr/connmanager.go	2	86.07%
txscript/taproot.go	2	95.93%
<!--	Total:	6

Totals
Change from base Build 2456385554:	-0.03%
Covered Lines:	26447
Relevant Lines:	48080

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💛 - Coveralls