Network transaction builder component

[Mirko-von-Leipzig]

Add a new component responsible for creating network network transactions from network notes.

See parent issue #692 for full details.

Communication with other components will happen via gRPC and should consist of:

1. gRPC server to receive
   1. network notes, and
   2. network tx committed / reverted
2. store client to fetch
   1. existing network notes on startup
   2. account state
3. block-producer client to submit network transactions
   Configuration should therefore be the three gRPC endpoints.

Network notes, transactions and accounts still have to be formerly defined in the protocol, so we can simply use the conventional existing types for now.

We can ignore actual execution for now, but if a simple algorithm presents itself then that is also welcome. Note that we should not bundle multiple notes yet.

The component should be initialized with the set of network notes from the store; this should occur before any components are started since otherwise we have additional synchronisation between components to perform.

Partially blocked by

• gRPC methods for network transactions #695