LoomProvider should use a dummy address when making static calls

[enlight]

Currently the LoomProvider API requires a private key to be passed into the constructor

loom-js/src/loom-provider.ts

Line 150 in b0dc03f

privateKey: Uint8Array,

Frequently users will be constructing a LoomProvider for use with eth-signing, in which case the private key isn't actually needed. The private key should be optional, if it's not provided a dummy address should be used.

A related problem is that if a dummy private key is specified when constructing LoomProvider and callerChainId is set (as is usually the case when doing eth-signing) then the address used for static calls has the callerChainId prefix - which is the wrong prefix since the private key is always expected to be a native ed25519 key, and therefore the chain ID should always match the client chain ID.

[eduardonunesp]

#307