Add `gettxspendingprevout` for `btcd` and fix version check

[yyforyongyu]

This PR adds the RPC gettxspendingprevout for btcd so we can do mempool lookups. This RPC is also ported to the rpcclient to replace the raw request made on the btcwallet side.

In addition, we now start to track btcd versions in rpcclient to make sure RPC updates are handled properly.

[coveralls]

Pull Request Test Coverage Report for Build 8077551462

Details

• -76 of 169 (55.03%) changed or added relevant lines in 7 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 56.753%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rpcclient/infrastructure.go	0	9	0.0%
rpcclient/backend_version.go	43	65	66.15%
rpcclient/rawtransactions.go	0	45	0.0%
<!--	Total:	93	169	55.03%	-->

Files with Coverage Reduction	New Missed Lines	%
rpcclient/infrastructure.go	1	39.21%
<!--	Total:	1		-->

Totals
Change from base Build 8057525961:	0.03%
Covered Lines:	29284
Relevant Lines:	51599

---

💛 - Coveralls

[saubyk]

cc: @Crypt-iQ for review

[saubyk]

cc: @guggero

[Chinwendu20]

Nice pr @yyforyongyu I like the progression.

question/suggestion: Do you think we should separate the implementations of the Backend version interface into separate files? like bitcoin.go and btcd.go in the same folder. In the future if we want to track more backends we just create a new file and that way it is easier to follow?

[Roasbeef]

Do you think we should separate the implementations of the Backend version interface into separate files? like bitcoin.go and btcd.go in the same folder. In the future if we want to track more backends we just create a new file and that way it is easier to follow?

@Chinwendu20, I think if we did that, we'd want to add a version prefix/suffix to the file names, so it's clear that this is code for parsing the versions, rather than a bitcoind specific implementation of something.