sample-tapd.conf+tapcfg: add default signet/testnet4 uni servers

[guggero]

Uses our universe servers as proof couriers by default for the networks testnet4 and signet.

NOTE: The required CNAME entries don't exist yet (but should soon), just putting this up so we don't forget for the final release.

[coveralls]

Pull Request Test Coverage Report for Build 15182918524

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 40 (0.0%) changed or added relevant lines in 1 file are covered.
• 21 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.02%) to 37.141%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tapcfg/server.go	0	40	0.0%

Files with Coverage Reduction	New Missed Lines	%
tappsbt/create.go	2	26.74%
asset/asset.go	3	45.46%
asset/mock.go	4	64.71%
tapdb/universe.go	4	80.4%
tapgarden/caretaker.go	8	68.31%

Totals
Change from base Build 15160950886:	-0.02%
Covered Lines:	26998
Relevant Lines:	72690

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💛 - Coveralls

[Roasbeef]

LGTM 🌈

[guggero]

The ports for testnet4 and signet actually need to be 443, since the infrastructure is set up slightly differently from the testnet and mainnet universes.