chore: PXE oracle refactor

[Thunkar]

Attempt to address PXE's structural problems, but mostly renaming:

• Too many things were called oracles. In this PR, only the foreign call handlers passed to the simulator during private and unconstrained executions are called oracles. As such, we have PrivateExecutionOracle (formerly ClientExecutionContext) and UnconstrainedExecutionOracle (formerly ViewDataOracle). The private execution oracle inherits from the unconstrained one, since that's how our execution model works (private can do all the things unconstrained can plus more)
• ContractDataOracle has been substituted by ContractDataProvider. In short, it's just a cache layer + merkle tree constructor that provides contract information to both the execution oracles and the kernel oracle, so I think it's a lot more accurate now.
• DBOracle interface is dead. Like the others, it's called now ExecutionDataProvider since it's a layer that the actual Private and Unconstrained oracles require to answer oracle calls, retrieving and sending info through it as necessary.
• As a consequence, the infamous SimulatorOracle is dead. It's replacement is called PXEDataProvider, implementing the ExecutionDataProvider interface the simulator oracles need to do their thing. This one just happens to be PXE's implementation, which receives PXE's db, the node and a bunch of other things in order to work.
• Speaking of the AztecNode, it's NO LONGER PROVIDED to the simulator oracles. All the data oracles require must be provided via the ExecutionDataProvider, which forces this interface to implement part of the node (but I think it's cleaner like this)

[benesjan]

Lovely improvement. Feel free to merge once you address my comments