a6b5ec18f rpc: creates possibility to preserve labels on importprivkey (marcoagner)
Pull request description:
Closes#13087.
As discussed in the issue, this is a feature request instead of a bug report since the behaviour was as intended (i.e. label with default: `''`). With this, the old behaviour is kept while the possibility to achieve the preservation of labels, as expected in the open issue, is added.
Tree-SHA512: b33be50e1e7f62f7ddfae953177ba0926e2d848961f9fac7501c2b513322c0cb95787745d07d137488267bad1104ecfdbe800c6747f94162eb07c976835c1386
