Fix manifest error when connecting to an Anvil dev network

[ericglau]

When handling network manifest files, we use the RPC method hardhat_metadata to determine if the connected network is a Hardhat development network, in which case we write the manifest to the temp dir instead of .openzeppelin.

Furthermore, if this method has a forkedNetwork field in its response, we treat this as a network fork and make a copy of the origin networks's manifest file, so that we can write to it for the fork without affecting the original version.

In recent versions of Anvil, Anvil also supports the hardhat_metadata RPC method, but has "forkedNetwork": null in its response as seen here.

This PR does two things:

1. Try calling anvil_metadata to determine if Anvil, and if so, use anvil for the manifest file prefix to more accurately reflect the network type.
2. Check if forkedNetwork is null or undefined, in which case we determine that it is not a network fork.

Fixes #943

[ernestognw]

Thanks @ericglau