Move urlDecode to wallet/rpcwallet.cpp, where it's used #14269

Remove now-unnecessary includes of httpserver.h from bitcoind.cpp and wallet/rpcwallet.cpp.

Pass decode_plus arg as an int rather than bool, as documented here: http://www.wangafu.net/~nickm/libevent-2.0/doxygen/html/http_8h.html#ac2914389616f04199aded10444fd8e42 https://github.com/libevent/libevent/blob/29cc8386a2f7911eaa9336692a2c5544d8b4734f/http.c#L3182

I think we should get rid of evhttp_uriencode and evhttp_uridecode and replace them with project local URI-encode/decode functions. Having them in-tree would make reviewing them easier. Generally I think it is a good idea to minimize the exposure surface to each third-party dependency by limiting the use of more general utility functions that happens to be provided by the dependency (such as evhttp_uriencode and evhttp_uridecode in libevent).

<!--e57a25ab6845829454e8d69fc972939a-->Note to reviewers: This pull request conflicts with the following ones:

• #13815 (util: Add [[nodiscard]] to all {Decode,Parse}... functions returning bool by practicalswift)
• #10973 (Refactor: separate wallet from node by ryanofsky)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

I tend towards NACK.

It's intentional that http* more or less wrap libevent_http. This would allow switching to a different HTTP server fairly easily, if that ever becomes necessary.

Do not spread use of those functions to other files.