feat: Add testnet4 chain support

[bullet-tooth]

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

Also, I have intention to add testnet4 support to btcwallet after this PR

[Roasbeef]

Looks like we have another here: #2275

Neither that one, nor this one, have any of the new difficulty adjustment algorithm changes.

[jcvernaleo]

I know that in #2275 they were working on the difficultly bits. @marcopeereboom I thought you had started on that part, is it in your PR?

[bullet-tooth]

@Roasbeef @jcvernaleo thanks for your inputs.
As I can see the #2275 is still in draft so if you could provide a little context about this "difficulty adjustment algorithm changes" I could implement it as well in this PR.

[Roasbeef]

@bullet-tooth see the BIP: https://bips.dev/94/

[coveralls]

Pull Request Test Coverage Report for Build 11982011907

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

• 13 of 41 (31.71%) changed or added relevant lines in 8 files are covered.
• 188 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.02%) to 57.219%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
peer/peer.go	0	1	0.0%
rpcclient/infrastructure.go	0	2	0.0%
rpcserver.go	0	2	0.0%
blockchain/difficulty.go	6	10	60.0%
config.go	0	4	0.0%
btcd.go	0	5	0.0%
blockchain/validate.go	5	15	33.33%

Files with Coverage Reduction	New Missed Lines	%
btcd.go	85	1.89%
config.go	103	5.07%

Totals
Change from base Build 11704113837:	-0.02%
Covered Lines:	29891
Relevant Lines:	52240

---

💛 - Coveralls

[bullet-tooth]

Hey @Roasbeef, do you have any estimates about reviewing/merging this PR?

[guggero]

If you clean up the PR into a review friendly commit structure, I'll take a look.
Here are some docs to go by: https://github.com/lightningnetwork/lnd/blob/master/docs/code_contribution_guidelines.md
and https://github.com/lightningnetwork/lnd/blob/master/docs/code_formatting_rules.md.
Those aren't strictly enforced in this project but definitely nice to have if you want review from the lnd team.