Add ExtendedPrivKey and ExtendPubKey classes

[elmarsan]

Hi!

The Kotlin library only contains ExtendedKeyInfo.
It would be very useful to also have ExtendedPrivKey and ExtendPubKey.

[notmandatory]

Hi, can you explain a bit more about how you want to use ExtendedPrivKey and ExtendPubKey if we expose them in the Kotlin library? It would help with prioritizing and maybe there's another way to do what you need.

[elmarsan]

I'm trying to avoid use xprv as wallet descriptor.
At this moment you don't have a way the get the public key, then it's hard to figure out from xprv without using other library.

Let me put an example:

val key = generateExtendedKey(Network.TESTNET, WordCount.WORDS12, null)

// Desired scenario
val wallet = Wallet(
        "wpkh([c258d2e4/84/1/0]${key.xpub}/0/*)",
        "wpkh([c258d2e4/84/1/0]${key.xpub}/1/*)",
        Network.TESTNET,
        DatabaseConfig.Memory,
        BlockchainConfig.Electrum(
            ElectrumConfig("ssl://electrum.blockstream.info:60002", null, 5u, null, 10u)
        )
    )

Currently there is no way to do that.
For example padawan wallet uses the xprv as wallet descriptor

[notmandatory]

OK yes I understand now and this is a very good suggestion. But to make your example work we don't need new ExtendedPrivKey and ExtendPubKey classes. Would it work for you if we add an xpub property to the ExtendedKeyInfo class?

The FFI definition would look like this:

dictionary ExtendedKeyInfo {
  string mnemonic;
  string xprv;
  string xpub;
  string fingerprint;
};

[elmarsan]

I suppose that adding just xpub it's what the biggest parts of developers would need it.

[notmandatory]

I'm open to other solutions if you have a need for something more. I just realized an existing PR #139 also adds xpub to ExtendedKeyInfo. If no objections I'm going to work on getting that one merged and then we also solve this issue.

[notmandatory]

This should be fixed as part of #154 which is focused on deriving xpubs from xprv keys.