feat: Add testnet4 chain support (recreated)

[bullet-tooth]

This PR is a recreation of the previous PR because I incidentally dropped a brunch and the PR was closed automatically.

Description

This PR adds support of the Bitcoin testnet version 4 chain (testnet3 is in a way of deprecation).

Reference: https://bips.dev/94/

Testnet4 genesis block: https://mempool.space/testnet4/block/00000000da84f2bafbbc53dee25a72ae507ff4914b867c565be350b0da8bf043

[guggero]

Thanks for the updates. Definitely looks better now.

Have a couple of fixes that are required, otherwise things crash quite immediately (please test things before requesting review in the future).

After those fixes I was able to sync up to block height 70513

[saubyk]

hey @bullet-tooth are you planning to work on this pr at a high priority? We are interested in moving this along quickly. Thanks.

[bullet-tooth]

hey @bullet-tooth are you planning to work on this pr at a high priority? We are interested in moving this along quickly. Thanks.

Done.

Also, verified locally, it could sync up to the current height (71627).

[coveralls]

Pull Request Test Coverage Report for Build 13524296390

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

• 10 of 68 (14.71%) changed or added relevant lines in 10 files are covered.
• 217 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-2.0%) to 55.27%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rpcclient/infrastructure.go	0	2	0.0%
rpcserver.go	0	2	0.0%
cmd/addblock/config.go	0	4	0.0%
cmd/findcheckpoint/config.go	0	4	0.0%
config.go	0	4	0.0%
database/cmd/dbtool/globalconfig.go	0	4	0.0%
blockchain/difficulty.go	2	7	28.57%
cmd/btcctl/config.go	0	10	0.0%
blockchain/validate.go	7	30	23.33%

Files with Coverage Reduction	New Missed Lines	%
btcutil/gcs/gcs.go	1	80.95%
mempool/mempool.go	1	66.67%
database/ffldb/db.go	5	91.23%
config.go	210	5.09%

Totals
Change from base Build 12775873016:	-2.0%
Covered Lines:	29949
Relevant Lines:	54187

---

💛 - Coveralls

[guggero]

Would be nice if you could squash/fixup the last 4 commits into those that actually introduced the code (to make it easy for the next reviewer, see https://thoughtbot.com/blog/git-interactive-rebase-squash-amend-rewriting-history).
Other than that looks good to me, thanks!

Need a second reviewer though.