Embedded wallet v2 scope planning

[jinglescode]

The existing embedded wallet is not ready for serious production us, and may likely to contain bugs. A new embedded wallet is needed.

[jinglescode]

high level issues with current design:

• super super messy
• have same interface for all kinds of keys (BIP32, ED25519)
• have one interface for all kinds of keys = bad, they should behave differently/separated
• BIP32 and ED25519 should be different class? they two are different

some objectives for new wallet:

• do key deviations
• do signing
• so people can implement custom utxo selection, some callback function
  • have some kind of bridge with txbuilder for selection
• extendable to support observable pattern, good when dealing with utxos set is big
  • if spend, will notify
  • some ways to deal with large utxo set
• need some way to update the "state" of the wallet, such as utxo set

@lisicky @twwu123 @abdelkrimdev

[jinglescode]

• define interface and extension points
• unit tested
• end to end test
• sort out crypto package