Make wallet.getContractInstance fetch contract instance from both PXE and AztecNode

[benesjan]

@porco-rosso-j reported the following:

Currently, PXE doesn't have an easy way to register ( import ) an existing contract that's been deployed in other PXEs. Contract instance, one of the pxe.registerContract()'s param, has to be manually reconstructed which requires several params such as, salt, constructor args, public keys, deployer wallet address, etc.

*pxe.getContractInstance() only returns instance stored in db if it's already been registered, or undefined.

Santiago proposed that the best solution would be to have the function on wallet request for the instance both in PXE and in AztecNode and return undefined only if the instance was not found in any of these.

I think this is a reasonable solution as it allows us to declutter PXE (currently in plenty of the endpoints we just forward the requests to aztec node which is not great).