Stellar - Send transactions

[Olemis Lang]

Feature description

Create Ethereum transactions

Is your feature request related to a problem? Please describe.
There is no way to create Ethereum transactions after #246

Describe the solution you'd like
Integrate Stellar transactions into simple and advanced wallet views

Possible implementation

• List Stellar Wallet addresses
• List unspent outputs by address
• Implement Transaction interface for Stellar
• Extend Transaction interface to cope with Stellar-specific features
• Serialize Stellar Transaction
• Implement transaction signing interfaces
• Implement Stellar blockchain transaction API for multi-wallet transactions
• Implement P2PKH in Stellar blockchain sign service for partial signing with multi-wallet transactions
• Implement Stellar contract-specific features