lnwallet+lnd: support watch-only initialization for lnd nodes

[Roasbeef]

This issue is related to this on going btcwallet PR which adds watch-only support to btcwallet as well as our issue tracking PSBT integration within lnd, and finally #3929 . Once those items are carried out lnd will be able to operate in a mode wherein it requires no private key material to be accessed within the main process. As a result, we'll need a slightly different initialization mode for lnd in order to support this new operating mode. As is, when creating a new lnd instance you need to either provide raw entropy which is used to derive a master private key, or an aezeed. What we need instead, is a new more which accepts a series of xpubs, which are them imported into the internal wallet.

Steps to Completion

• Add a new set of RPC calls to the WalletKit sub-server that allows a user to export a given xpub based on a target key family.

• This new watch-only variant will then need to interact with each of the yet-to-be-finalized APIs in the btcwallet PR referenced above to import each of the xpubs for each key family into the wallet.

• Extend the existing InitWalllet RPC call to support importing a set of base xpubs: one for each of our currently used active key families. Note that we need to import each xpub rather than a single one derived from the main seed as each key family or account uses hardened derivation for everything "above" it. #5486

Note that we'll need to complete #3929 before this issue can be fully implemented.

[Roasbeef]

btcsuite/btcwallet#667 has been merged now, which unblocks this PR once #3929 is in, although they can be worked on concurrently.

[Roasbeef]

Keeping this open for now as the second point to extend InitWallet to allow the relevant key families ti be passed in isn't yet resolved. This is the missing gap that would allow stuff like lnd just having the raw account xpubs for the multi-sig keys with signing being done in a remote/detached manner.

[Roasbeef]

Closing this as the final item is being tracked in a new issue now.