rpcclient: implement getaddressinfo command

[onyb]

Docs	getaddressinfo

Fields such as label, and labels->purpose are not included, since they are deprecated, and will be removed in Bitcoin Core 0.21.

Most of the fields in the results struct are repeated in the embedded field if (for example) the address itself is a script embedding another address. To have minimal code duplication, I have factored them out to a private struct.

Another noteworthy thing in this PR is the custom unmarshalling I did to support loading a script to a txscript.ScriptClass, without duplicating the entire struct with raw fields. I learnt about this technique in this article if you want to read.

Tested locally with Bitcoin Core v0.20.1. See rpcclient/example_test.go for details.

[coveralls]

Pull Request Test Coverage Report for Build 256438448

• 58 of 80 (72.5%) changed or added relevant lines in 4 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.7%) to 53.587%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
btcjson/walletsvrresults.go	45	49	91.84%
rpcclient/wallet.go	0	18	0.0%

Files with Coverage Reduction	New Missed Lines	%
peer/peer.go	3	75.39%
database/ffldb/blockio.go	4	92.62%

Totals
Change from base Build 253997810:	0.7%
Covered Lines:	20713
Relevant Lines:	38653

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💛 - Coveralls

[jcvernaleo]

OK