bb822a7af8 wallet, rpc: add listdescriptors private option (S3RK)
Pull request description:
Rationale: make it possible to backup your wallet with `listdescriptors` command
* The default behaviour is still to show public version
* For private version only the root xprv is returned
Example use-case:
```
> bitcoin-cli -regtest -named createwallet wallet_name=old descriptors=true
> bitcoin-cli -regtest -rpcwallet=old listdescriptors true | jq '.descriptors' > descriptors.txt
> bitcoin-cli -regtest -named createwallet wallet_name=new descriptors=true blank=true
> bitcoin-cli -regtest -rpcwallet=new importdescriptors "$(cat descriptors.txt)"
```
In case of watch-only wallet without private keys there will be following output:
```
error code: -4
error message:
Can't get descriptor string.
```
ACKs for top commit:
achow101:
re-ACK bb822a7af8
Rspigler:
tACK bb822a7af8
jonatack:
ACK bb822a7af8 per `git diff 2854ddc bb822a7`
prayank23:
tACK bb822a7af8
meshcollider:
Code review ACK bb822a7af8
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lsilva01